Flexural anatomy is frequently described as a difficult area. It is simplified when each flexure is considered as being centered on a mesenteric contiguity. The ileocaecal flexure arises at the point where the ileum is continuous with the caecum around the ileocaecal mesenteric flexure. Similarly, the  hepatic flexure  is formed between the right mesocolon and transverse mesocolon at the mesenteric confluence. The colonic component of the hepatic flexure is draped around this mesenteric confluence. Furthermore, the  splenic flexure  is formed by the mesenteric confluence between the transverse and left mesocolon. The colonic component of the splenic flexure occurs lateral to the mesenteric confluence. At every flexure, a continuous peritoneal fold lies outside the colonic/mesocolic complex tethering this to the posterior abdominal wall. [ 2 ] [ 6 ]


The  transverse mesocolon  is that section of the mesentery attached to the  transverse colon  that lies between the  colic flexures .


The  sigmoid mesocolon  is that region of the mesentery to which the  sigmoid colon  is attached at the gastrointestinal mesenteric margin.


The  mesoappendix  is the portion of the mesentery connecting the  ileum  to the  appendix . It may extend to the tip of the appendix. It encloses the  appendicular artery  and vein, as well as  lymphatic  vessels, nerves, and often a  lymph node .


The  mesorectum  is that part attached to the upper third of the rectum.


Understanding the macroscopic structure of the mesenteric organ meant that associated structures—the peritoneal folds and congenital and omental adhesions—could be better appraised. The small intestinal mesenteric fold occurs where the small intestinal mesentery folds onto the posterior abdominal wall and continues laterally as the right mesocolon. During mobilization of the small intestinal mesentery from the posterior abdominal wall, this fold is incised, allowing access to the interface between the small intestinal mesentery and the retroperitoneum. The fold continues at the inferolateral boundary of the ileocaecal junction and turn  cephalad  as the right paracolic peritoneal fold. This fold is divided during lateral to medial mobilization, permitting the surgeon to serially lift the right colon and associated mesentery off the underlying fascia and retroperitoneum. At the hepatic flexure, the right lateral peritoneal fold turns and continues medially as the hepatocolic peritoneal fold. Division of the fold in this location permits separation of the colonic component of the hepatic flexure and mesocolon off the retroperitoneum. [ 2 ] [ 6 ]


Interposed between the hepatic and splenic flexures, the greater omentum adheres to the transverse colon along a further band or fold of peritoneum. Dissection through this allows access to the cephalad (top) surface of the transverse mesocolon. Focal adhesions frequently tether the greater omentum to the cephalad aspect of the transverse mesocolon. The left colon is associated with a similar anatomic configuration of peritoneal folds; the splenic peritoneal fold is contiguous with the left lateral paracolic peritoneal fold at the splenic flexure. Division of the latter similarly allows for the separation of the left colon and associated mesentery off the underlying fascia and frees it from the retroperitoneum. The left lateral paracolic peritoneal fold continues distally at the lateral aspect of the mobile component of the mesosigmoid. [ 2 ] [ 6 ]


Determination of the macroscopic structure of the mesenteric organ allowed a recent characterisation of the histological and electron microscopic properties. [ 7 ]  The microscopic structure of the mesocolon and associated fascia is consistent from ileocecal to mesorectal levels. A surface  mesothelium  and underlying connective tissue is universally apparent.  Adipocytes  lobules within the body of the mesocolon are separated by fibrous  septa  arising from submesothelial connective tissue. Where apposed to the retroperitoneum, two mesothelial layers separate the mesocolon and underlying retroperitoneum. Between these is  Toldt's fascia , a discrete layer of connective tissue. Lymphatic channels are evident in mesocolic connective tissue and in Toldt's fascia. [ 7 ]


The  primitive gut  is suspended from the posterior abdominal wall by the  dorsal mesentery . The gastrointestinal tract and associated dorsal mesentery are subdivided into  foregut ,  midgut , and  hindgut  regions based on the respective blood supply. The foregut is supplied by the  celiac trunk , the midgut is supplied by the  superior mesenteric artery  (SMA), and the hindgut is supplied by the  inferior mesenteric artery  (IMA). This division is established by the fourth week of  development . After this, the midgut undergoes a period of rapid elongation, forcing it to herniate through the  navel . During herniation, the midgut  rotates  90° anti-clockwise around the axis of the SMA and forms the midgut loop. The cranial portion of the loop moves to the right and the caudal portion of the loop moves toward the left. This  rotation  occurs at about the eighth week of development. The  cranial  portion of the loop will develop into the jejunum and most of the ileum, while the  caudal  part of the loop eventually forms the terminal portion of the ileum, the  ascending colon  and the initial two-thirds of the transverse colon. As the foetus grows larger, the mid-gut loop is drawn back through the umbilicus and undergoes a further 180° rotation, completing a total of 270° rotation. At this point, about 10 weeks, the  caecum  lies close to the  liver . From here it moves in a cranial to caudal direction to eventually lie in the lower right portion of the abdominal cavity. This process brings the ascending colon to lie vertically in the lateral right portion of the abdominal cavity apposed to the posterior abdominal wall. The descending colon occupies a similar position on the left side. [ 8 ] [ 9 ]


During these topographic changes, the dorsal mesentery undergoes corresponding changes. Most anatomical and embryological textbooks say that after adopting a final position, the ascending and descending mesocolons disappear during embryogenesis.  Embryology—An Illustrated Colour Text , "most of the mid-gut retains the original dorsal mesentery, though parts of the duodenum derived from the mid-gut do not. The mesentery associated with the ascending colon and descending colon is resorbed, bringing these parts of the colon into close contact with the body wall." [ 9 ]  In  The Developing Human , the author states, "the mesentery of the ascending colon fuses with the parietal peritoneum on this wall and disappears; consequently the ascending colon also becomes retroperitoneal". [ 10 ]  To reconcile these differences, several theories of embryologic mesenteric development—including the "regression" and "sliding" theories—have been proposed, but none has been widely accepted. [ 9 ] [ 10 ]


The portion of the  dorsal mesentery  that attaches to the  greater curvature  of the  stomach , is known as the  dorsal mesogastrium . The part of the dorsal mesentery that suspends the  colon  is termed the  mesocolon . The dorsal mesogastrium develops into the  greater omentum .


The development of the  septum transversum  takes part in the formation of the  diaphragm , while the caudal portion into which the  liver  grows forms the  ventral mesentery . The part of the ventral mesentery that attaches to the  stomach  is known as the  ventral mesogastrium . [ 11 ]


The  lesser omentum  is formed, by a thinning of the  mesoderm  or ventral mesogastrium, which attaches the stomach and  duodenum  to the anterior  abdominal wall . By the subsequent growth of the liver, this leaf of mesoderm is divided into two parts – the  lesser omentum  between the stomach and liver, and the  falciform  and  coronary ligaments  between the liver and the abdominal wall and diaphragm. [ 11 ]


In the adult, the ventral mesentery is the part of the  peritoneum  closest to the  navel .


Clarifications of the mesenteric anatomy have facilitated a clearer understanding of diseases involving the mesentery, examples of which include  malrotation  and  Crohn's disease  (CD). In CD, the mesentery is frequently thickened, rendering  hemostasis  challenging. In addition, fat wrapping—creeping fat—involves extension of mesenteric fat over the circumference of contiguous gastrointestinal tract, and this may indicate increased mesothelial plasticity. The relationship between mesenteric derangements and mucosal manifestations in CD points to a pathobiological overlap; some authors say that CD is mainly a mesenteric disorder that secondarily affects the GIT and systemic circulation. [ 12 ]


Thrombosis  of the  superior mesenteric vein  can cause  mesenteric ischemia  also known as  ischemic bowel . Mesenteric ischemia can also result from the formation of a  volvulus , a twisted loop of the small intestine that when it wraps around itself and also encloses the mesentery too tightly can cause  ischemia . [ 13 ]


The rationalization of mesenteric and peritoneal fold anatomy permits the surgeon to differentiate both from intraperitoneal adhesions—also called congenital adhesions. These are highly variable among patients and occur in several locations. Congenital adhesions occur between the lateral aspect of the peritoneum overlying the mobile component of the mesosigmoid and the parietal peritoneum in the left iliac fossa. During the lateral to the medial approach of mobilizing of the mesosigmoid, these must be divided first before the peritoneum proper can be accessed. Similarly, focal adhesions occur between the undersurface of the greater omentum and the cephalad aspect of the transverse mesocolon. These can be accessed after dividing the peritoneal fold that links the greater omentum and transverse colon. Adhesions here must be divided to separate the greater omentum off the transverse mesocolon, thus allowing access to the lesser sac proper. [ 2 ] [ 14 ]


While the total mesorectal excision (TME) operation has become the surgical gold standard for the management of rectal cancer, this is not so for colon cancer. [ 2 ] [ 14 ]
Recently, the surgical principles underpinning TME in rectal cancer have been extrapolated to colonic surgery. [ 15 ] [ 16 ]  Total or complete mesocolic excision (CME), use planar surgery and extensive mesenterectomy (high tie) to minimise breach of the mesentery and maximise lymph nodes yield. Application of this T/CME reduces local five-year recurrence rates in colon cancer from 6.5% to 3.6%, while cancer-related five-year survival rates in patients resected for cure increased from 82.1% to 89.1%. [ 17 ]


Recent radiologic appraisals of the mesenteric organ have been conducted in the context of the contemporary understanding of mesenteric organ anatomy. When this organ is divided into non-flexural and flexural regions, these can readily be differentiated in most patients on CT imaging. Clarification of the radiological appearance of the human mesentery resonates with the suggestions of Dodds and enables a clearer conceptualization of mesenteric derangements in disease states. [ 18 ]  This is of immediate relevance in the spread of cancer from colon cancer and perforated diverticular disease, and in pancreatitis where fluid collections in the lesser sac dissect the mesocolon from the retroperitoneum and thereby extend distally within the latter. [ 19 ]


Mesentery has been known for thousands of years, however it was unclear whether mesentery is a single organ or there are several mesenteries. [ 20 ] [ better source needed ]  The classical anatomical description of the mesocolon is credited to British surgeon  Sir Frederick Treves  in 1885, [ 21 ]  although a description of the membrane as a single structure dates back to at least  Leonardo da Vinci . [ 22 ]  Treves is known for performing the first  appendectomy  in England in 1888; he was surgeon to both  Queen Victoria  and  King Edward VII . [ 23 ]  He studied the human mesentery and peritoneal folds in 100 cadavers and described the right and left mesocolons as vestigial or absent in the human adult. Accordingly, the small intestinal mesentery, transverse, and sigmoid mesocolons all terminated or attached at their insertions into the posterior abdominal wall. [ 21 ] [ 23 ]  These assertions were included in mainstream surgical, anatomical, embryological, and radiologic literature for more than a century. [ 24 ] [ 25 ]


Almost 10 years before Treves, the Austrian anatomist  Carl Toldt  described the persistence of all portions of the mesocolon into adulthood. [ 26 ]   Toldt was professor of anatomy in Prague and Vienna; he published his account of the human mesentery in 1879. Toldt identified a fascial plane between the mesocolon and the underlying retroperitoneum, formed by the fusion of the visceral peritoneum of the mesocolon with the parietal peritoneum of the retroperitoneum; this later became known as Toldt's fascia. [ 26 ] [ 27 ]


In 1942, anatomist  Edward Congdon  also demonstrated that the
right and left mesocolons persisted into adulthood and remained separate from the retroperitoneum—extraretroperitoneal. [ 28 ]  Radiologist  Wylie J. Dodds  described this concept in 1986. [ 18 ]  Dodds extrapolated that unless the mesocolon remained an extraretroperitoneal structure—separate from the retroperitoneum—only then would the radiologic appearance of the mesentery and peritoneal folds be reconciled with actual anatomy. [ 18 ]


Descriptions of the mesocolon by Toldt, Congdon, and Dodds have largely been ignored in mainstream literature until recently. A formal appraisal of the mesenteric organ anatomy was conducted in 2012; it echoed the findings of Toldt, Congdon, and Dodds. [ 6 ]  The single greatest advance in this regard was the identification of the mesenteric organ as being contiguous, as it spans the gastrointestinal tract from duodenojejunal flexure to mesorectal level. [ 6 ]


In 2012 it was discovered that the mesentery was a single organ, which precipitated advancement in colon and rectum surgery [ 29 ]  and in sciences related to  anatomy  and  development .


The word "mesentery" and its  Neo-Latin  equivalent  mesenterium  ( / ˌ m ɛ z ə n ˈ t ɛ r i ə m / ) use the  combining forms   mes-  +  enteron , ultimately from ancient Greek  μεσέντερον  ( mesenteron ), from  μέσος  ( mésos , "middle") +  ἔντερον  ( énteron , "gut"), yielding "mid-intestine" or "midgut". The adjectival form is "mesenteric" ( / ˌ m ɛ z ə n ˈ t ɛ r ɪ k / ).


An improved understanding of mesenteric structure and histology has enabled a formal characterization of mesenteric lymphangiology. [ 7 ]   Stereologic  assessments of the lymphatic vessels demonstrate a rich lymphatic network embedded within the mesenteric connective tissue lattice. On average, vessels occur every 0.14 mm (0.0055 in), and within 0.1 mm (0.0039 in) from the mesocolic surfaces—anterior and posterior. Lymphatic channels have also been identified in Toldt's fascia, though the significance of this is unknown. [ 7 ]


( Wayback Machine  copy)


Methanogens  are a group of  microorganisms  that produce  methane  as a byproduct of their metabolism. They play an important role in the digestive system of  ruminants . The  digestive tract  of ruminants contains four major parts:  rumen ,  reticulum ,  omasum  and  abomasum . The food with saliva first passes to the rumen for breaking into smaller particles and then moves to the reticulum, where the food is broken into further smaller particles. Any indigestible particles are sent back to the rumen for  rechewing . The majority of  anaerobic microbes  assisting the cellulose breakdown occupy the rumen and initiate the fermentation process. The animal absorbs the fatty acids, vitamins and nutrient content on passing the partially digested food from the rumen to the omasum. This decreases the pH level and initiates the release of enzymes for further breakdown of the food which later passes to the abomasum to absorb remaining nutrients before excretion. This process takes about 9–12 hours.


Some of the microbes in the ruminant digestive system are:


Migrating motor complex , also known as  migrating myoelectric complex ,  migratory motor complex ,  migratory myoelectric complex  and  MMC , is a cyclic, recurring motility pattern that occurs in the stomach and small bowel during  fasting ; it is interrupted by  feeding . [ 1 ]  A pattern of electrical activity is also observed in the  gastrointestinal tract  in a regular cycle during this interdigestive period.


MMC was discovered and characterized in fasting dogs in 1969 by Dr. Joseph H. Szurszewski at the  Mayo Clinic . [ 2 ]  He also showed that this activity stops upon eating a meal, and suggested that it induces a motor activity that acts as an "interdigestive housekeeper" in the small intestine. [ 2 ]  These motor complexes trigger  peristaltic waves , which facilitate transportation of indigestible substances such as  bone ,  fiber , and  foreign bodies  from the  stomach , through the  small intestine , past the  ileocecal sphincter , and into the  colon . MMC activity varies widely across individuals and within an individual when measured on different days. The MMC occurs every 90–230 minutes during the interdigestive phase (i.e., between meals) and is responsible for the  rumbling  experienced when hungry. [ 3 ] [ 4 ]  It also serves to transport  bacteria  from the small intestine to the large intestine and to inhibit the migration of colonic bacteria into the  terminal ileum ; an impairment to the MMC typically results in  small intestinal bacterial overgrowth . [ 5 ]


The MMC originates mostly in the stomach—although ~25% will arise from the  duodenum  or proximal  jejunum —and can travel to the distal end of the  ileum . [ 6 ]  They consist of four distinct phases:


Movements of the small bowel are believed to be controlled by the central and enteric nervous systems, intestinal muscles, and numerous peptides and hormones. For example, the MMC is thought to be initiated by  motilin , and it does not directly depend on extrinsic nerves. [ 7 ]  Additionally, gastrin, insulin, cholecystokinin, glucagon, and secretin have been reported to disrupt the MMC.


Eating interrupts the MMC. For example, one study found that a continental breakfast of 450 Kcal causes the MMC to disappear for 213 ± 48 minutes. [ 8 ]  The number of calories and nature of food determine the length of the disruption with fats causing a longer disruption than carbohydrates which in turn cause a longer disruption than protein. [ 9 ]


Most of the cleaning waves in the MMC happen at night while we are asleep. For many people this will be sufficient to help maintain a healthy, balanced environment in the digestive tract. For others, it may be beneficial to space out food intake to allow for a couple cleaning waves to occur between meals throughout the day as well. [ 10 ]


Autoimmunity following infection by a pathogen producing  CdtB , such as  C. jejuni , may be the leading cause of MMC impairment. [ 11 ]   Narcotics  are also known to impair the MMC. [ 12 ]  Stress has been shown to reduce MMC activity as well. [ 13 ]


Patients with  SIBO  and  IBS  have on average a third as many MMC phase III events with those events being roughly 30% shorter on average. [ 14 ]


Drugs used to enhance gastrointestinal motility are generally referred to as  prokinetics . Serotonin induces phase III of the MMC, and so serotonin receptor agonists are commonly administered as prokinetics. [ 15 ]  Motilin administration causes phase III contractions, and so motilin agonists are another common prokinetic. [ 16 ]


Eradication of bacterial overgrowth has been shown to partially restore MMC activity. [ 14 ]


An  elemental diet  has been hypothesized to partially restore MMC function. [ 17 ]


A  monogastric  organism has a simple single-chambered  stomach  (one stomach). Examples of monogastric  omnivores  include  humans ,  pigs ,  hamsters  and  rats . Furthermore, there are monogastric  carnivores  such as  cats . [ 1 ]  A monogastric organism is contrasted with  ruminant  organisms (which have four-chambered complex stomachs), such as  cattle ,  goats , and  sheep .  Herbivores with monogastric digestion can digest  cellulose  in their diets by way of symbiotic gut  bacteria . However, their ability to extract energy from cellulose digestion is less efficient than in ruminants. [ 2 ]


Herbivores digest cellulose by microbial   fermentation . Monogastric herbivores which can digest  cellulose  nearly as well as ruminants are called  hindgut fermenters , while ruminants are called  foregut fermenters . [ 3 ]   These are subdivided into two groups based on the relative size of various digestive organs in relationship to the rest of the system:  colonic  fermenters tend to be larger species such as horses and  rhinos , and  cecal  fermenters are smaller animals such as rabbits and rodents. [ 4 ]   Great apes  derive significant amounts of  phytanic acid  from the  hindgut fermentation  of plant materials. [ 5 ]


Monogastrics cannot digest the fiber molecule cellulose as efficiently as ruminants, though the ability to digest cellulose varies amongst species. [ 2 ]


A monogastric digestive system works as soon as the food enters the mouth.  Saliva  moistens the food and begins the digestive process. (Note that horses have no (or negligible amounts of)  amylase  in their saliva). After being swallowed, the food passes from the  esophagus  into the stomach, where  stomach acid  and  enzymes  help to break down the food. Once food leaves the stomach and enters the small intestine, the  pancreas  secretes enzymes and alkali to neutralize the stomach acid.


This  biology  article is a  stub . You can help Wikipedia by  expanding it .


The  mouth  is the  body orifice  through which many  animals   ingest  food and  vocalize . The  body cavity  immediately behind the mouth opening, known as the  oral cavity  (or  cavum oris  in  Latin ), [ 2 ]  is also the first part of the  alimentary canal , which leads to the  pharynx  and the  gullet . In  tetrapod   vertebrates , the mouth is bounded on the outside by the  lips  and  cheeks  — thus the oral cavity is also known as the  buccal cavity  (from Latin  bucca , meaning "cheek") [ 3 ]  — and contains the  tongue  on the inside. Except for some groups like  birds  and  lissamphibians , vertebrates usually have  teeth  in their mouths, [ 4 ]  although some  fish  species have  pharyngeal teeth  instead of oral teeth.


Most  bilaterian   phyla , including  arthropods ,  molluscs  and  chordates , have a two-opening  gut tube  with a mouth at one end and an  anus  at the other. Which end forms first in  ontogeny  is a criterion used to classify bilaterian animals into  protostomes  and  deuterostomes .


In the first  multicellular animals , there was probably no mouth or gut and food particles were engulfed by the cells on the exterior surface by a process known as  endocytosis . The particles became enclosed in  vacuoles  into which enzymes were secreted and  digestion  took place  intracellularly . The digestive products were absorbed into the cytoplasm and diffused into other cells. This form of digestion is used nowadays by simple organisms such as  Amoeba  and  Paramecium  and also by  sponges  which, despite their large size, have no mouth or gut and capture their food by endocytosis. [ 5 ]


However,  most animals  have a mouth and a gut, the lining of which is continuous with the  epithelial cells  on the surface of the body. A few animals which live parasitically originally had guts but have secondarily lost these structures. The original gut of  diploblastic  animals probably consisted of a mouth and a one-way gut. Some modern invertebrates still have such a system: food being ingested through the mouth, partially broken down by enzymes secreted in the gut, and the resulting particles engulfed by the other cells in the gut lining. Indigestible waste is ejected through the mouth. [ 5 ]


In animals at least as complex as an  earthworm , the  embryo  forms a dent on one side, the  blastopore , which deepens to become the  archenteron , the first phase in the formation of the  gut . In deuterostomes, the blastopore becomes the anus while the gut eventually tunnels through to make another opening, which forms the mouth. In the protostomes, it used to be thought that the blastopore formed the mouth ( proto–  meaning "first") while the anus formed later as an opening made by the other end of the gut. More recent research, however, shows that in protostomes the edges of the slit-like blastopore close up in the middle, leaving openings at both ends that become the mouth and anus. [ 6 ]


Apart from sponges and  placozoans , almost all animals have an internal gut cavity, which is lined with gastrodermal cells. In less advanced invertebrates such as the  sea anemone , the mouth also acts as an anus. Circular muscles around the mouth are able to relax or contract in order to open or close it. A fringe of tentacles thrusts food into the cavity and it can gape widely enough to accommodate large prey items. Food passes first into a  pharynx  and digestion occurs extracellularly in the  gastrovascular cavity . [ 7 ]   Annelids  have simple tube-like guts, and the possession of an anus allows them to separate the digestion of their foodstuffs from the absorption of the nutrients. [ 8 ]
Many molluscs have a  radula , which is used to scrape microscopic particles off surfaces. [ 9 ]  In invertebrates with hard exoskeletons, various mouthparts may be involved in feeding behaviour. Insects have a range of mouthparts suited to their mode of feeding. These include mandibles, maxillae and labium and can be modified into suitable appendages for chewing, cutting, piercing, sponging and sucking. [ 10 ]   Decapods  have six pairs of mouth appendages, one pair of mandibles, two pairs of maxillae and three of  maxillipeds . [ 11 ]  Sea urchins have a set of five sharp calcareous plates, which are used as jaws and are known as  Aristotle's lantern . [ 12 ]


In vertebrates, the first part of the digestive system is the  buccal cavity , commonly known as the mouth. The buccal cavity of a fish is separated from the opercular cavity by the  gills . Water flows in through the mouth, passes over the gills and exits via the  operculum  or  gill slits . Nearly all fish have  jaws  and may seize food with them but most feed by opening their jaws, expanding their pharynx and sucking in food items. The food may be held or chewed by teeth located in the jaws, on the roof of the mouth, on the pharynx or on the  gill arches . [ 13 ]


Nearly all amphibians are carnivorous as adults. Many catch their prey by flicking out an elongated tongue with a sticky tip and drawing it back into the mouth, where they hold the prey with their jaws. They then swallow their food whole without much chewing. [ 14 ]  They typically have many small hinged  pedicellate teeth , the bases of which are attached to the jaws, while the crowns break off at intervals and are replaced. Most amphibians have one or two rows of teeth in both jaws but some frogs lack teeth in the lower jaw. In many amphibians, there are also  vomerine teeth  attached to the bone in the roof of the mouth. [ 15 ]


The mouths of reptiles are largely similar to those of mammals. The  crocodilians  are the only reptiles to have teeth anchored in  sockets  in their jaws. [ 16 ]  They are able to replace each of their approximately 80 teeth up to 50 times during their lives. [ 17 ]  Most reptiles are either carnivorous or insectivorous, but turtles are often herbivorous. Lacking teeth that are suitable for efficiently chewing of their food, turtles often have  gastroliths  in their stomach to further grind the plant material. [ 18 ]  Snakes have a very flexible lower jaw, the two halves of which are not rigidly attached, and numerous other joints in their skull. These modifications allow them to open their mouths wide enough to swallow their prey whole, even if it is wider than they are. [ 19 ]


Birds do not have teeth, relying instead on other means of gripping and macerating their food. Their  beaks  have a range of sizes and shapes according to their diet and are composed of elongated mandibles. The upper mandible may have a nasofrontal hinge allowing the beak to open wider than would otherwise be possible. The exterior surface of beaks is composed of a thin, horny sheath of  keratin . [ 20 ]  Nectar feeders such as  hummingbirds  have specially adapted brushy tongues for sucking up nectar from flowers. [ 21 ]


In mammals, the buccal cavity is typically roofed by the  hard  and  soft palates , floored by the  tongue  and surrounded by the  cheeks ,  salivary glands , and upper and lower  teeth . The upper teeth are embedded in the  upper jaw  and the lower teeth in the  lower jaw , which articulates with the  temporal bones  of the  skull . The  lips  are soft and fleshy folds which shape the entrance into the mouth. The buccal cavity empties through the  pharynx  into the  oesophagus . [ 22 ]


Crocodilians  living in the tropics can gape with their mouths to provide cooling by  evaporation  from the mouth lining. [ 23 ]  Some mammals rely on panting for  thermoregulation  as it increases evaporation of water across the moist surfaces of the lungs, the tongue and mouth. Birds also avoid overheating by gular fluttering, flapping the wings near the gular (throat) skin, similar to panting in mammals. [ 24 ]


Various animals use their mouths in threat displays. They may gape widely, exhibit their teeth prominently, or flash the startling colours of the mouth lining. This display allows each potential combatant an opportunity to assess the weapons of their opponent and lessens the likelihood of actual combat being necessary. [ 25 ]


A number of species of bird use a  gaping , open beak in their fear and threat displays. Some augment the display by hissing or breathing heavily, while others clap their beaks. [ 26 ]


Mouths are also used as part of the mechanism for producing sounds for communication. To produce sounds, air is forced from the lungs over  vocal cords  in the larynx. In humans, the pharynx, soft palate, hard palate,  alveolar ridge , tongue, teeth and lips are termed  articulators  and play their part in the production of  speech . Varying the position of the tongue in relation to the other articulators or moving the lips restricts the airflow from the lungs in different ways and changes the mouth's resonating properties, producing a range of different sounds. [ 27 ]  In frogs, the sounds can be amplified using sacs in the throat region. The vocal sacs can be inflated and deflated and act as resonators to transfer the sound to the outside world. [ 28 ]  A bird's song is produced by the flow of air over a vocal organ at the base of the  trachea , the  syrinx . For each burst of song, the bird opens its beak and closes it again afterwards. The beak may move slightly and may contribute to the resonance but the song originates elsewhere. [ 29 ]


Mucoid plaque  (or mucoid cap or rope) is a  pseudoscientific  term used by some  alternative medicine  advocates to describe what is claimed to be a combination of harmful  mucus-like  material and food residue that they say coats the  gastrointestinal tract  of most people. The term was coined by Richard Anderson, a  naturopath  and  entrepreneur , who sells a range of products that claim to "cleanse" the body of such purported plaques. [ 1 ]


Many such " colon cleansing " products are promoted to the public on websites that have been described as making misleading medical claims. [ 2 ]  The presence of laxatives,  bentonite clay , and fibrous thickening agents in some of these "cleansing agents" has led to suggestions that the products themselves produce the excreted matter regarded as the plaque. [ 2 ] [ 3 ]
The concept of a 'mucoid plaque' has been dismissed by medical experts as having no anatomical or physiological basis. [ 4 ] [ 5 ] [ 6 ]


Various forms of colon cleansing were popular in the 19th and early 20th century. [ 7 ]  In 1932, Bastedo wrote in the  Journal of the American Medical Association  about his observation of mucus masses being removed during a colon irrigation procedure: "When one sees the dirty gray, brown or blackish sheets, strings and rolled up wormlike masses of tough mucus with a rotten or dead-fish odor that are obtained by colon irrigations, one does not wonder that these patients feel ill and that they obtain relief and show improvement as the result of the irrigation." [ 8 ]


While colonic irrigation enjoyed a vogue in the early 20th century as a possible cure for numerous diseases, subsequent research showed that it was useless and potentially harmful. [ 9 ]  With the scientific rationale for "colon cleansing" disproven, the idea fell into disrepute as a form of  quackery , with a 2005 medical review stating that "there is no evidence to support this ill-conceived theory that has been long abandoned by the scientific community." [ 10 ]   Similarly, in response to claims that colon cleansing removes "toxins", Bennett Roth, a gastroenterologist at the  University of California , stated that "there is absolutely no science to this whatsoever. There is no such thing as getting rid of quote-unquote 'toxins.' The colon was made to carry stool. This is total baloney." [ 11 ]  The preoccupation with such bowel management products has been described as a "quaint and amusing chapter in the history of weird medical beliefs." [ 12 ]  Nevertheless, interest in colonic " autointoxication " as a cause of illness, and in colonic irrigation as a cure, enjoyed a revival in  alternative medicine  at the end of the 20th century. [ 9 ]


The term "mucoid plaque" was coined and popularized by  naturopath  and  entrepreneur  Richard Anderson, who sells a range of products that claim to cleanse the body of such purported plaques by causing them to be eliminated. [ 1 ]   Anderson describes a mucoid plaque as a rubbery, ropey, generally green  gel -like mucus film that covers the  epithelial cells  of the  hollow organs , particularly of the  alimentary canal . Anderson also claims the plaque can impair digestion and the absorption of nutrients, hold  pathogens , and cause illnesses such as  diarrhea ,  bowel cancer ,  allergies  and skin conditions. Based on these claims, he promotes efforts to remove the plaque, and sells a range of products to this end. [ 3 ] [ 13 ]


Though Anderson argues that his beliefs are backed by scientific research, his claims are primarily supported by  anecdotal evidence  rather than  empirical data , and doctors have noted the absence of mucoid plaques. Anderson claims this is due to medical textbooks failing to cover the concept, which results in doctors not knowing what to look for. [ 1 ]


Practicing  physicians  have dismissed the concept of mucoid plaque as a  hoax  and a "non-credible concept". [ 4 ]  A pathologist at the  University of Texas  School of Medicine addressed Anderson's claims directly, saying that he has "seen several thousand intestinal biopsies and have never seen any 'mucoid plaque.' This is a complete fabrication with no anatomic basis." [ 1 ] [ 5 ]


Another pathologist, Edward Friedlander, noted that, in his experience, he has never observed anything resembling a "toxic bowel settlement", and that some online photographs actually depict what he recognises as a  blood clot . [ 6 ]  Commenting on claims that waste material can adhere to the colon, Douglas Pleskow, a gastroenterologist at  Beth Israel Deaconess Medical Center , stated, "that is the urban legend. In reality, most people clear their GI tract within three days." [ 11 ]


In a review of websites promoting products that claim to remove 'mucoid rope' or plaque from consumers' intestines, Howard Hochster of  New York University  wrote that these websites are "abundant, quasi-scientific, and unfortunately convincing to a biologically uneducated public." He noted that although such sites are entertaining, they are disturbing in that they promote a belief that has no basis in physiology. [ 2 ]


Hochster also noted that a preparation marketed to remove mucoid plaque contains laxatives and bulky fibrous ingredients. Thus, the rope-like fecal material expelled from people who consume this product "certainly is a result of the figs and senna in this preparation," rather than any sort of pathologic 'plaque'. [ 2 ]  Other 'colon cleanser' products contain bentonite clay that, when ingested, would also result in production of bulky stools. [ 3 ]


In many cases, customers purchase supplement products that are said to help the body excrete the so-called 'mucoid plaque'. The customer may consume a number of pills, and then within 12–48 hours, will pass a rope-like fecal material in their subsequent bowel movements. This fecal material is said to be the 'mucoid plaque'. However, analysis of supplements consumed by the customer shows that the active ingredient is very similar to that of clay used in clumping cat litter. This clay takes a negative mould of the large intestine which is then excreted during the customer's next bowel movement.  [ 14 ]


The  muscularis mucosae  (or  lamina muscularis mucosae ) is a thin layer ( lamina ) of  muscle  of the  gastrointestinal tract , located outside the  lamina propria , and separating it from the  submucosa . It is present in a continuous fashion from the  esophagus  to the upper  rectum  (the exact nomenclature of the rectum's muscle layers is still being debated). A discontinuous muscularis mucosae–like muscle layer is present in the  urinary tract , from the  renal pelvis  to the  bladder ; as it is discontinuous, it should not be regarded as a true muscularis mucosae.


In the  gastrointestinal tract , the term  mucosa  or  mucous membrane  refers to the combination of  epithelium , lamina propria, and (where it occurs) muscularis mucosae. [ 1 ]   The etymology suggests this, since the Latin names translate to "the mucosa's own special layer" ( lamina propria mucosae ) and "muscular layer of the mucosa" ( lamina muscularis mucosae ).


The muscularis mucosae is composed of several thin layers of  smooth muscle fibers  oriented in different ways which keep the mucosal surface and underlying  glands  in a constant state of gentle agitation to expel contents of glandular  crypts  and enhance contact between epithelium and the contents of the  lumen .


Stacey E. Mills — Histology for Pathologists: 3rd (third) Edition, page 670.  ISBN   9780781762410


Nitrosamines  (or more formally  N -nitrosamines ) are  organic compounds  produced by industrial processes. [ 1 ]


The  chemical structure  is  R 2 N−N=O , where R is usually an  alkyl group . [ 2 ]  They feature a  nitroso  group ( NO + ) that are "probable human carcinogens", [ 3 ]  bonded to a deprotonated  amine . Most nitrosamines are  carcinogenic  in animals. [ 4 ]  A 2006 systematic review supports a "positive association between nitrite and nitrosamine intake and  gastric cancer , between meat and processed meat intake and gastric cancer and  oesophageal cancer , and between preserved fish, vegetable and smoked food intake and gastric cancer, but is not conclusive". [ 5 ]


The organic chemistry of nitrosamines is well developed with regard to their syntheses, their structures, and their reactions. [ 7 ] [ 8 ]  They usually are produced by the reaction of  nitrous acid  ( HNO 2 ) and secondary amines, although other  nitrosyl  sources (e.g.  N 2 O 4 ,  NOCl ,  RONO ) have the same effect: [ 9 ]


The nitrous acid usually arises from protonation of a  nitrite . This synthesis method is relevant to the generation of nitrosamines under some biological conditions. [ 10 ]   The nitrosation is also reversible, particularly in acidic solutions of  nucleophiles . [ 11 ]   Aryl  nitrosamines rearrange to give a  para -nitroso aryl amine in the  Fischer-Hepp rearrangement . [ 12 ]


With regards to structure, the  C 2 N 2 O  core of nitrosamines is planar, as established by  X-ray crystallography . The N-N and N-O distances are 132 and 126 pm, respectively in  dimethylnitrosamine , [ 13 ]  one of the simplest members of a large class of N-nitrosamines


Nitrosamines are not directly carcinogenic. Metabolic activation is required to convert them to the  alkylating agents  that modify bases in DNA, inducing mutations. The specific alkylating agents vary with the nitrosamine, but all are proposed to feature  alkyldiazonium  centers. [ 14 ] [ 6 ]


In 1956, two British scientists, John Barnes and Peter Magee, reported that a simple member of the large class of N-nitrosamines,  dimethylnitrosamine , produced liver  tumours  in rats. Subsequent studies showed that approximately 90% of the 300 nitrosamines tested were  carcinogenic  in a wide variety of animals. [ 15 ]


A common way ordinary consumers are exposed to nitrosamines is through tobacco use and cigarette smoke. [ 14 ]   Tobacco-specific nitrosamines  also can be found in American  dip snuff ,  chewing tobacco , and to a much lesser degree,  snus  (127.9  ppm  for American dip snuff compared to 2.8 ppm in Swedish snuff or snus). [ 16 ]


Nitroso  compounds react with  primary amines  in acidic environments to form  nitrosamines , which human metabolism converts to mutagenic  diazo compounds .  Small amounts of nitro and nitroso compounds form during meat  curing ; the toxicity of these compounds  preserves  the meat against  bacterial infection .  After curing completes, the concentration of these compounds appears to degrade over time.  Their presence in finished products has been tightly regulated since several food-poisoning cases in the early 20th century, [ 17 ]  but consumption of large quantities of processed meats can still cause a slight elevation in  gastric  and  oesophageal cancer  risk today. [ 18 ] [ 19 ] [ 20 ] [ 21 ]


For example, during the 1970s, certain  Norwegian  farm animals began exhibiting elevated levels of  liver cancer .  These animals had been fed  herring   meal  preserved with  sodium nitrite . The sodium nitrite had reacted with  dimethylamine  in the fish and produced  dimethylnitrosamine . [ 22 ]


The effects of nitroso compounds vary dramatically across the gastrointestinal tract, and with diet.  Nitroso compounds present in stool do not induce nitrosamine formation, because stool has neutral  pH . [ 23 ] [ 24 ]   Stomach acid  catalyzes nitrosamine compound formation and is the main location of the reaction during digestion. [ 25 ]


The formation process is inhibited when amine concentration is low (e.g. a low-protein diet or no fermented food).  The process may also be inhibited in the case of high  vitamin C (ascorbic acid)  or  erythorbic acid [ 26 ]  concentration (e.g. high-fruit diet). [ 27 ] [ 28 ] [ 29 ]  However, when 10% of the meal is fat, the effect reverses, and ascorbic acid markedly increases nitrosamine formation. [ 25 ] [ 30 ]  Vitamin C and erythorbic acid are already commonly used in the meat industry because they enhance the binding of nitrite to myoglobin, encouraging the formation of the desired pink color. [ 31 ]


There have been recalls for various medications due to the presence of nitrosamine impurities. There have been recalls for  angiotensin II receptor blockers ,  ranitidine ,  valsartan , Duloxetine, and others.


The US  Food and Drug Administration  published guidance about the control of nitrosamine impurities in medicines. [ 32 ] [ 33 ]   Health Canada  published guidance about nitrosamine impurities in medications [ 34 ]  and a list of established acceptable intake limits of nitrosamine impurities in medications. [ 35 ]


In biochemistry,  nitrosamines  are a class of compounds that can form during  food   digestion .  The presence of their precursors,  nitrites , in  cured meats , is controversial, because of a small connection to cancer risk.


Nitroso  compounds react with  primary amines  in acidic environments to form  nitrosamines , which human metabolism converts to mutagenic  diazo compounds .  Small amounts of nitro and nitroso compounds form during meat  curing ; the toxicity of these compounds  preserves  the meat against  bacterial infection .  After curing completes, the concentration of these compounds appears to degrade over time.  Their presence in finished products has been tightly regulated since several food-poisoning cases in the early 20th century, [ 1 ]  but consumption of large quantities of processed meats can still cause a slight elevation in  gastric  and  oesophageal cancer  risk today. [ 2 ] [ 3 ] [ 4 ] [ 5 ]


For example, during the 1970s, certain  Norwegian  farm animals began exhibiting elevated levels of  liver cancer .  These animals had been fed  herring   meal  preserved with  sodium nitrite . The sodium nitrite had reacted with  dimethylamine  in the fish and produced  dimethylnitrosamine . [ 6 ]


The effects of nitroso compounds vary dramatically across the gastrointestinal tract, and with diet.  Nitroso compounds present in stool do not induce nitrosamine formation, because stool has neutral  pH . [ 7 ] [ 8 ]   Stomach acid  catalyzes nitrosamine compound formation and is the main location of the reaction during digestion. [ 9 ]


The formation process is inhibited when amine concentration is low (e.g. a low-protein diet or no fermented food).  The process may also be inhibited in the case of high  vitamin C (ascorbic acid)  or  erythorbic acid [ 10 ]  concentration (e.g. high-fruit diet). [ 11 ] [ 12 ] [ 13 ]  However, when 10% of the meal is fat, the effect reverses, and ascorbic acid markedly increases nitrosamine formation. [ 9 ] [ 14 ]  Vitamin C and erythorbic acid are already commonly used in the meat industry because they enhance the binding of nitrite to myoglobin, encouraging the formation of the desired pink color. [ 15 ]


In  organic chemistry ,  nitroso  refers to a  functional group  in which the  nitric oxide  ( −N=O ) group is attached to an organic  moiety . As such, various nitroso groups can be categorized as  C -nitroso compounds (e.g., nitroso alkanes ;  R−N=O ),  S -nitroso compounds ( nitrosothiols ;  RS−N=O ),  N -nitroso compounds (e.g.,  nitrosamines ,  RN(−R’)−N=O ), and  O -nitroso compounds ( alkyl nitrites ;  RO−N=O ).


Nitroso compounds can be prepared by the reduction of  nitro compounds [ 1 ]  or by the oxidation of  hydroxylamines . [ 2 ]
Ortho-nitrosophenols may be produced by the  Baudisch reaction . In the  Fischer–Hepp rearrangement , aromatic 4-nitrosoanilines are prepared from the corresponding  nitrosamines .


Nitrosoarenes typically participate in a  monomer–dimer equilibrium .  The azobenzene  N , N'- dioxide (Ar( – O)N + = + N(O – )Ar) dimers, which are often pale yellow, are generally favored in the solid state, whereas the deep-green monomers are favored in dilute solution or at higher temperatures.  They exist as  cis  and  trans  isomers . [ 4 ]   The central "double bond" in the dimer in fact has a bond order of about 1.5. [ 5 ]


When stored in  protic media ,  primary  and  secondary  nitrosoalkanes  isomerize  to  oximes . [ 6 ]   Some tertiary nitrosoalkanes also isomerize to oximes through C-C bond fission, particularly if the bond is  electron-poor . [ 7 ]   Nitrosophenols and naphthols isomerize to the oxime  quinone  in solution, but reversibly; nitrosophenol ethers typically dealkylate to facilitate the isomerization.  Nitroso tertiary anilines generally do not dealkylate in that way. [ 8 ]


Due to the stability of the nitric oxide  free radical , nitroso organyls tend to have very low C–N  bond dissociation energies : nitrosoalkanes have BDEs on the order of 30–40 kcal/mol (130–170 kJ/mol), while nitrosoarenes have BDEs on the order of 50–60 kcal/mol (210–250 kJ/mol).  As a consequence, they are generally heat- and light-sensitive.  Compounds containing O–(NO) or N–(NO) bonds generally have even lower bond dissociation energies.  For instance,  N -nitrosodiphenylamine , Ph 2 N–N=O, has a N–N bond dissociation energy of only 23 kcal/mol (96 kJ/mol). [ 9 ]


Organonitroso compounds serve as a  ligands  giving  transition metal nitroso complexes . [ 10 ]


Many reactions make use of an intermediate nitroso compound, such as the  Barton reaction  and  Davis–Beirut reaction , as well as the synthesis of  indoles , for example:  Baeyer–Emmerling indole synthesis ,  Bartoli indole synthesis . In the  Saville reaction , mercury is used to replace a nitrosyl from a thiol group.


C -nitroso compounds are used in organic synthesis as synthons in some well-documented chemical reactions such as hetero Diels-Alder (HDA), nitroso-ene and nitroso-aldol reactions. [ 11 ]


Nitrosyls are non-organic compounds containing the NO group, for example directly bound to the metal via the N atom, giving a metal–NO moiety. Alternatively, a  nonmetal  example is the common reagent  nitrosyl chloride  ( Cl−N=O ). Nitric oxide is a stable  radical , having an unpaired electron. Reduction of nitric oxide gives the nitrosyl  anion ,  NO − :


Oxidation of NO yields the  nitrosonium   cation ,  NO + :


Nitric oxide can serve as a  ligand  forming  metal nitrosyl complexes  or just metal nitrosyls. These complexes can be viewed as adducts of  NO + ,  NO − , or some intermediate case.


Nitroso  compounds react with  primary amines  in acidic environments to form  nitrosamines , which human metabolism converts to mutagenic  diazo compounds .  Small amounts of nitro and nitroso compounds form during meat  curing ; the toxicity of these compounds  preserves  the meat against  bacterial infection .  After curing completes, the concentration of these compounds appears to degrade over time.  Their presence in finished products has been tightly regulated since several food-poisoning cases in the early 20th century, [ 12 ]  but consumption of large quantities of processed meats can still cause a slight elevation in  gastric  and  oesophageal cancer  risk today. [ 13 ] [ 14 ] [ 15 ] [ 16 ]


For example, during the 1970s, certain  Norwegian  farm animals began exhibiting elevated levels of  liver cancer .  These animals had been fed  herring   meal  preserved with  sodium nitrite . The sodium nitrite had reacted with  dimethylamine  in the fish and produced  dimethylnitrosamine . [ 17 ]


The effects of nitroso compounds vary dramatically across the gastrointestinal tract, and with diet.  Nitroso compounds present in stool do not induce nitrosamine formation, because stool has neutral  pH . [ 18 ] [ 19 ]   Stomach acid  catalyzes nitrosamine compound formation and is the main location of the reaction during digestion. [ 20 ]


The formation process is inhibited when amine concentration is low (e.g. a low-protein diet or no fermented food).  The process may also be inhibited in the case of high  vitamin C (ascorbic acid)  or  erythorbic acid [ 21 ]  concentration (e.g. high-fruit diet). [ 22 ] [ 23 ] [ 24 ]  However, when 10% of the meal is fat, the effect reverses, and ascorbic acid markedly increases nitrosamine formation. [ 20 ] [ 25 ]  Vitamin C and erythorbic acid are already commonly used in the meat industry because they enhance the binding of nitrite to myoglobin, encouraging the formation of the desired pink color. [ 26 ]


The   omasum , also known as the  bible , [ 1 ]  the  fardel , [ 1 ]  the  manyplies [ 1 ]  and the  psalterium , [ 1 ]  is the third compartment of the  stomach  in  ruminants . The omasum comes after the  rumen  and  reticulum  and before the  abomasum . Different ruminants have different omasum structures and function based on the food that they eat and how they developed through evolution. [ 2 ]


The omasum can be found on the right side of the cranial portion of the rumen. [ 3 ]  It receives food from the  reticulum  through the reticulo-omasal orifice [ 3 ]  and provides food to the abomasum through the omaso-abomasal orifice. [ 4 ]   The omasum is spherical [ 5 ]  to crescent shape [ 6 ]  and has multiple leaflets similar to that of a book [ 7 ]  called omasal laminae. [ 4 ]   The omasal laminae are made of thin muscular layers covered with a nonglandular mucous membrane. [ 4 ]  The omasal laminae come from the sides of the large curvature and project towards the inside of the omasum, extending from the reticulo-omasal orifice to the omaso-abomasal orifice. [ 8 ] [ 4 ]  They greatly increase the surface area of the omasum. [ 3 ] [ 9 ]  The laminae are covered in omasal papillae that are claw-like in some ruminants or blunt cones in others. [ 4 ] [ 2 ]  These papillae further increase the surface area but they also provide increased friction against the food particles. [ 3 ]


The function of the omasum is not completely understood. [ 5 ]  During the second contraction phase of the reticulum,  the reticule-omasal sphincter opens for a few seconds allowing a small volume of finely dispersed and well-fermented ingesta to enter the omasum. [ 3 ]


The omasum has two physiological compartments: omasal canal that transfers food from the reticulum to the omasum, and the inter-laminate recesses between the mucosal laminae which provide the area for absorption. [ 2 ]  The omasum is where food particles that are small enough get transferred into the abomasum for enzymatic digestion. [ 5 ] [ 2 ]  In ruminants with a more sophisticated omasum [ example needed ] , the large surface area [ 9 ]  allows it to play a key role in the absorption of water,  electrolytes , [ 2 ] [ 4 ]  volatile fatty acids, minerals, and the fermentation of food. [ 5 ]


Young ruminants that are still drinking milk have an esophageal groove that allows milk to bypass the rumen and go straight from the esophagus to the omasum. [ 10 ]


An early version of the omasum is seen in early ruminants like  duikers  and  muntjacs , where it is a little more than a strainer sieve which prevents un-chewed foods from entering the  abomasum . [ 2 ]


The smallest omasum belongs to ruminants that consume high quality diets like the  moose  and  roe deer , while the largest belongs to those who are un-selective grass and roughage eaters like cattle and sheep. [ 2 ]


The omasum is not only bigger in grass and roughage eaters but there is greater differentiation in the book-like structure; seen as an increase in the number of laminae. [ 2 ]


The  pectinate line  ( dentate line ) is a line which divides the upper two-thirds and lower third of the  anal canal .  Developmentally, this line represents the  hindgut - proctodeum  junction.


It is an important anatomical landmark in humans, and forms the boundary between the anal canal and the  rectum  according to the anatomic definition. [ 1 ]  Colorectal surgeons instead define the anal canal as the zone from the anal verge to the anorectal ring (palpable structure formed by the  external anal sphincter  and the  puborectalis muscle ). [ 1 ]  Several distinctions can be made based upon the location of a structure relative to the pectinate line:


Peristalsis  ( / ˌ p ɛr ɪ ˈ s t æ l s ɪ s /   PERR -ih- STAL -siss ,  US  also  /- ˈ s t ɔː l -/   - STAWL - ) [ 1 ]  is a  type of  intestinal motility , characterized by  radially symmetrical  contraction and relaxation of  muscles  that propagate in a  wave  down a tube, in an  anterograde  direction. Peristalsis is progression of coordinated contraction of involuntary circular muscles, which is preceded by a simultaneous contraction of the longitudinal muscle and relaxation of the circular muscle in the lining of the gut. [ 2 ]


In much of a  digestive tract , such as the human gastrointestinal tract,  smooth muscle tissue  contracts in sequence to produce a peristaltic wave, which propels a ball of  food  (called a  bolus  before being transformed into  chyme  in the stomach) along the tract. The peristaltic movement comprises relaxation of circular smooth muscles, then their contraction behind the chewed material to keep it from moving backward, then longitudinal contraction to push it forward.


Earthworms  use a similar mechanism to drive their locomotion, [ 3 ] [ self-published source? ]  and some modern machinery imitate this design. [ 4 ]


The word comes from  Neo-Latin  and is derived from the  Greek   peristellein , "to wrap around," from  peri -, "around" +  stellein , "draw in, bring together; set in order". [ 5 ]


Peristalsis is generally directed caudal, that is, towards the  anus . This sense of direction might be attributable to the polarisation of the  myenteric plexus . Because of the reliance of the peristaltic reflex on the myenteric plexus, it is also referred to as the myenteric reflex. [ 6 ]


The food bolus causes a stretch of the gut  smooth muscle  that causes  serotonin  to be secreted to sensory neurons, which then get activated. These sensory neurons, in turn, activate neurons of the  myenteric plexus , which then proceed to split into two  cholinergic  pathways: a retrograde and an anterograde. Activated neurons of the retrograde pathway release  substance molecules alsoP  and  acetylcholine  to contract the smooth muscle behind the bolus. The activated neurons of the anterograde pathway instead release  nitric oxide  and  vasoactive intestinal polypeptide  to relax the smooth muscle caudal to the bolus. This allows the food bolus to effectively be pushed forward along the digestive tract. [ 7 ]


After food is chewed into a bolus, it is  swallowed  and moved through the esophagus. Smooth muscles contract behind the bolus to prevent it from being squeezed back into the mouth. Then rhythmic, unidirectional waves of contractions work to rapidly force the food into the stomach. The  migrating motor complex  (MMC) helps trigger peristaltic waves. This process works in one direction only, and its sole esophageal function is to move food from the mouth into the stomach (the MMC also functions to clear out remaining food in the stomach to the small bowel and remaining particles in the small bowel into the colon). [ 8 ]


In the esophagus, two types of peristalsis occur:


During  vomiting , the propulsion of food up the esophagus and out the mouth comes from the contraction of the  abdominal muscles ; peristalsis does not reverse in the esophagus. [ citation needed ]


When a peristaltic wave reaches at the end of the  esophagus , the  cardiac sphincter  (gastroesophageal sphincter) opens, allowing the passage of bolus into the stomach. The gastroesophageal sphincter normally remains closed and does not allow the stomach's food contents to move back. The churning movements of the stomach's thick muscular wall blend the food thoroughly with the acidic  gastric juice , producing a mixture called the  chyme . The  muscularis layer  of the stomach is thickest and maximum peristalsis occurs here. After short intervals, the  pyloric sphincter  keeps on opening and closing so the chyme is fed into the intestine in installments.


Once processed and digested by the stomach, the semifluid chyme is passed through the  pyloric sphincter  into the  small intestine . Once past the stomach, a typical peristaltic wave lasts only a few seconds, traveling at only a few centimeters per second. Its primary purpose is to mix the chyme in the intestine rather than to move it forward in the intestine. Through this process of mixing and continued digestion and absorption of nutrients, the chyme gradually works its way through the small intestine to the  large intestine . [ 8 ]


In contrast to peristalsis,  segmentation contractions  result in that churning and mixing without pushing materials further down the digestive tract.


Although the  large intestine  has peristalsis of the type that the small intestine uses, it is not the primary propulsion. Instead, general contractions called mass action contractions occur one to three times per day in the large intestine, propelling the chyme (now feces) toward the rectum. Mass movements often tend to be triggered by meals, as the presence of chyme in the stomach and duodenum prompts them ( gastrocolic reflex ). Minimum peristalsis is found in the  rectum  part of the large intestine as a result of the thinnest  muscularis layer .


The human  lymphatic system  has no central pump. Instead,  lymph   circulates  through peristalsis in the lymph capillaries as well as valves in the capillaries, compression during contraction of adjacent skeletal muscle, and arterial pulsation.


During  ejaculation , the smooth muscle in the walls of the  vasa deferentia  contract reflexively in peristalsis, propelling  sperm  from the  testicles  to the  urethra . [ 10 ]


The  earthworm  is a limbless  annelid  worm with a  hydrostatic skeleton  that moves by peristalsis. Its hydrostatic skeleton consists of a fluid-filled body cavity surrounded by an extensible body wall. The worm moves by radially constricting the anterior portion of its body, increasing length via hydrostatic pressure. This constricted region propagates posteriorly along the worm's body. As a result, each segment is extended forward, then relaxes and re-contacts the substrate, with hair-like  setae  preventing backward slipping. [ 11 ]  Various other invertebrates, such as  caterpillars  and  millipedes , also move by peristalsis.


A  peristaltic pump  is a positive-displacement pump in which a motor pinches advancing portions of a flexible tube to propel a fluid within the tube. The pump isolates the fluid from the machinery, which is important if the fluid is abrasive or must remain sterile.


Robots have been designed that use peristalsis to achieve locomotion, as the earthworm uses it. [ 12 ] [ 13 ]


The  peritrophic matrix  (from the prefix  peri- , meaning around, and  -trophic , referring to nutrition(food)) or  peritrophic membrane  is a semi-permeable, non-cellular structure which surrounds the food  bolus  in an organism's  midgut . Although they are often found in  insects , [ 1 ]  peritrophic matrixes are found in seven different  phyla . [ 2 ]  The peritrophic matrix serves several functions, including improvement of  digestion , protection against mechanical and chemical damage and serving as a barrier to infection by  pathogens . [ 3 ]   Such peritrophic envelopes are also of great ecological importance in marine environments.


The peritrophic matrix is composed of regularly arranged  chitin  microfibrils, (3–13% of matrix mass), and species specific  proteins  (20–55%) embedded in a  proteoglycan  matrix. [ 4 ] [ 5 ]  The peritrophic matrix also includes very small pores which allow for passage of small  molecules  into and out of the matrix. Thus, due to size limitations (pores reach a maximum size of 10 nm) larger, unwanted materials taken in while feeding are trapped and  excreted  along with the matrix. [ 3 ]


Type I formation of a peritrophic matrix is thought to be the  ancestral  method, and is found in the majority of  organisms  that produce a peritrophic matrix. In type I formation, the peritrophic matrix is secreted by the entire midgut, and is formed simply by delamination from the surface of the midgut  epithelium . [ 3 ]  Type I formation usually occurs as a response to  feeding , but can also be produced continually. When formed in response to feeding, a single matrix is secreted by the midgut epithelium. This matrix surrounds the food bolus and is later excreted along with unwanted materials present in the food bolus after digestion. When formed continually, as in the insect family  Acrididae  (locusts), multiple peritrophic matrixes are secreted and surround the food bolus, creating a peritrophic envelope. When no food bolus is present, peritrophic matrixes that are secreted are rapidly passed in excrement. [ 6 ]


Type II formation of a peritrophic matrix is considered to be a  derived  technique, and is found only in some families of the  diptera ,  dermaptera ,  embioptera  and  lepidoptera  orders of insects. [ 3 ]  In type II formation, the peritrophic matrix is produced by a specialized group of cells present on the  proventriculus  of the anterior midgut. [ 6 ]  Type II formation is a continuous process which is carried out regardless of the presence or absence of a food bolus. Thus, the peritrophic matrix is secreted as an unbroken, concentric, “sleeve like” structure. Although the peritrophic matrix is secreted continually, the presence of a food bolus significantly increases the rate of production. In addition, the presence of a food bolus stimulates the production of multiple matrices which surround the bolus. Following the secretion of a primary peritrophic matrix, subsequent matrices are secreted underneath the first matrix to create a layered peritrophic envelope. [ 3 ]


In many organisms the primary function of the peritrophic matrix is to improve  digestion . Following feeding, the food bolus is surrounded by the peritrophic matrix, effectively isolating it from the  midgut   epithelium . This isolation creates two distinct compartments within the midgut, the endoperitrophic space and ectoperitrophic space. This compartmentalization of the midgut provides three general advantages: prevention of non-specific binding of undigested material to the epithelium wall, conservation and concentration of  enzymes  and  substrates  and rapid removal of indigestible molecules. [ 7 ]


Prevention of non-specific binding is particularly important, as it increases the efficiency of the  absorption  process by filtering out undigested material which would otherwise block access to the midgut  epithelium . Due to the small pore size of the matrix, only small molecules which have been broken down by enzymes or can already be effectively absorbed come in contact with the midgut epithelium. The remaining materials, undigested food and unwanted molecules, are kept within the matrix until they can be broken down by  enzymes  or excreted. [ 7 ]


Concentrating enzymes and food substrate within the endoperitrophic space significantly decreases the time required for digestion in the midgut. In addition, since enzymes are small enough to easily move into and out of the peritrophic matrix, they are rarely lost when the matrix, along with its contents still in the endoperitrophic space, is excreted. A counterflow of fluid in the ectoperitrophic space also helps recycle enzymes, thus maximizing their efficacy. [ 7 ]


The presence of a peritrophic matrix significantly simplifies the excretion process. Rather than having to continually sift through a mixture of digestible and unwanted molecules, digestible molecules are quickly broken down by enzymes, removed from the matrix and absorbed. Once the digestive process is completed, unwanted molecules are kept confined within the endoperitrophic space and excreted along with the matrix. [ 7 ]


Although the peritrophic matrix is a very thin layer of compounds (type I matrixes reach a maximum thickness of 20 μm, type II matrixes reach a maximum thickness of 2 μm), it can withstand mechanical  pressure  strains up to 500 mmH2O. [ 3 ]  This capacity to expand prevents the food bolus from rupturing the delicate epithelial layer while assisting the passage of food through the gut.


Much like indigestible molecules present in the food bolus, many  toxins  are too large to pass through the small pores of the peritrophic matrix. For example, some insects that are resistant to the  insecticide   DDT  shed large amounts of the toxin in the peritrophic matrix. In addition, some smaller toxins bind with specific surface proteins present in the peritrophic matrix. This binding is particularly important for blood-feeding insects. Heme groups, which are components of  hemoglobin , an oxygen carrying protein present in vertebrate blood, act as strong  oxidizers  in insects. Although this  oxidizing agent  is safe in vertebrates, it is very damaging to insects. However,  heme  groups ingested in a blood meal bind to proteins on the peritrophic matrix, enabling insects to safely feed on blood. [ 8 ]


Organisms that take in food often infected with pathogens, such as  blood-feeding insects , also depend on the peritrophic matrix to filter out the disease agents, which are often too large to fit through the matrix pores. This benefit in particular is thought to be an important driving force in the evolution of peritrophic matrices, as many insects feeding on foods with low pathogen levels lack the ability to produce a peritrophic matrix. This trend is highlighted by  mosquitoes , as blood-feeding female mosquitoes produce a peritrophic matrix while nectar-feeding males do not. [ 9 ]  A significant trend can also be observed in the type of peritrophic matrix produced by blood-feeding insects that are capable of transmitting disease. The majority of blood-feeding insects that are good disease  vectors  produce a type I matrix. In comparison, blood-feeding insects that produce a type II matrix, which provides a more impenetrable barrier to pathogens, are rarely disease vectors.


Many pathogens are too large to fit through the small pores of the peritrophic matrix, and thus have evolved specialized mechanisms of evading being filtered out by the matrix. Since type I peritrophic matrixes are secreted in response to the presence of a food bolus in the midgut, some pathogens simply invade the epithelial cells before the matrix is excreted. Many helminth  microfilaria  and  arboviruses  (arthropod borne viruses) are transmitted to the mosquito in their infective form and are able to immediately invade mosquito tissue. [ 9 ]  However, other pathogens such as the malaria protozoan must first develop into an infective stage within the midgut before invading other tissues. These pathogens secrete  chitinase  and  proteinase  enzymes which dissolve the chitin microfibrils and proteins present in the peritrophic matrix. These enzymes open large holes in the membrane, allowing the pathogen to infect the epithelium and other tissues in the insect. [ 8 ]


The  pharynx  ( pl. :  pharynges ) is the part of the  throat  behind the  mouth  and  nasal cavity , and above the  esophagus  and  trachea  (the tubes going down to the  stomach  and the  lungs  respectively). It is found in vertebrates and invertebrates, though its structure varies across species. The pharynx carries food to the esophagus and air to the  larynx . The flap of cartilage called the  epiglottis  stops food from entering the larynx.


In humans, the pharynx is part of the  digestive system  and the  conducting zone  of the  respiratory system . (The conducting zone—which also includes the  nostrils  of the  nose , the  larynx ,  trachea ,  bronchi , and  bronchioles —filters, warms and moistens air and conducts it into the  lungs ). [ 1 ]  The human pharynx is conventionally divided into three sections: the  nasopharynx ,  oropharynx , and  laryngopharynx .


In humans, two sets of  pharyngeal muscles  form the pharynx and determine the shape of its  lumen . They are arranged as an inner layer of longitudinal muscles and an outer circular layer.


The upper portion of the pharynx, the nasopharynx, extends from the base of the  skull  to the upper surface of the  soft palate . [ 2 ]  It includes the space between the  internal nares  and the soft palate and lies above the oral cavity. The  adenoids , also known as the pharyngeal tonsils, are  lymphoid tissue  structures located in the posterior wall of the nasopharynx.  Waldeyer's tonsillar ring  is an annular arrangement of lymphoid tissue in both the nasopharynx and oropharynx. The nasopharynx is lined by  respiratory epithelium  that is pseudostratified, columnar, and ciliated.


Polyps  or  mucus  can obstruct the nasopharynx, as can congestion due to an upper respiratory infection. The  auditory tube , which connects the middle ear to the pharynx, opens into the nasopharynx at the pharyngeal opening of the auditory tube. The opening and closing of the auditory tubes serves to equalize the barometric pressure in the middle ear with that of the ambient atmosphere.


The anterior aspect of the nasopharynx communicates through the  choanae  with the nasal cavities. On its lateral wall is the  pharyngeal opening of the auditory tube , somewhat triangular in shape and bounded behind by a firm prominence, the  torus tubarius  or cushion, caused by the medial end of the cartilage of the tube that elevates the  mucous membrane .
Two folds arise from the cartilaginous opening:


The oropharynx lies behind the oral cavity, extending from the  uvula  to the level of the  hyoid bone . It opens anteriorly, through the  isthmus faucium , into the mouth, while in its lateral wall, between the  palatoglossal arch  and the  palatopharyngeal arch , is the  palatine tonsil . [ 4 ]  The anterior wall consists of the base of the tongue and the  epiglottic vallecula ; the lateral wall is made up of the tonsil, tonsillar fossa, and tonsillar (faucial) pillars; the superior wall consists of the inferior surface of the soft palate and the uvula. Because both food and air pass through the pharynx, a flap of connective tissue called the  epiglottis  closes over the  glottis  when food is swallowed to prevent  aspiration . The oropharynx is lined by non-keratinized squamous stratified epithelium.


The  HACEK  organisms ( H aemophilus ,  A ctinobacillus actinomycetemcomitans ,  C ardiobacterium hominis ,  E ikenella corrodens ,  K ingella ) are part of the normal oropharyngeal flora, which grow slowly, prefer a carbon dioxide-enriched atmosphere, and share an enhanced capacity to produce  endocardial  infections, especially in young children. [ 5 ]   Fusobacterium  is a pathogen. [ 6 ]


The laryngopharynx, ( Latin :  pars laryngea pharyngis ), also known as  hypopharynx , is the  caudal  part of the pharynx; it is the part of the throat that connects to the esophagus. It lies inferior to the epiglottis and extends to the location where this common pathway diverges into the respiratory ( laryngeal ) and digestive ( esophageal ) pathways. At that point, the laryngopharynx is continuous with the esophagus posteriorly. The esophagus conducts food and fluids to the stomach; air enters the larynx anteriorly. During swallowing, food has the "right of way", and air passage temporarily stops. Corresponding roughly to the area located between the 4th and 6th  cervical vertebrae , the superior boundary of the laryngopharynx is at the level of the  hyoid bone . The laryngopharynx includes three major sites: the  pyriform sinus , postcricoid area, and the posterior pharyngeal wall. Like the oropharynx above it, the laryngopharynx serves as a passageway for food and air and is lined with a  stratified squamous epithelium . It is innervated by the  pharyngeal plexus  and by the  recurrent laryngeal nerve .


The vascular supply to the laryngopharynx includes the  superior thyroid artery , the  lingual artery  and the  ascending pharyngeal artery . The primary neural supply is from both the  vagus  and  glossopharyngeal  nerves. The vagus nerve provides an  auricular branch  also termed "Arnold's nerve" which also supplies the external auditory canal, thus laryngopharyngeal cancer can result in referred  ear pain . This nerve is also responsible for the  ear-cough reflex  in which stimulation of the ear canal results in a person coughing.


The pharynx moves food from the mouth to the esophagus. It also moves air from the  nasal  and  oral  cavities to the  larynx . It is also used in human speech, as  pharyngeal consonants  are articulated here, and it acts as a  resonating chamber  during phonation.


Inflammation of the pharynx, or  pharyngitis , is the painful inflammation of the throat.


Pharyngeal cancer  is a cancer that originates in the neck and/or throat.


Waldeyer's tonsillar ring  is an anatomical term collectively describing the annular arrangement of lymphoid tissue in the pharynx. Waldeyer's ring circumscribes the naso- and oropharynx, with some of its tonsillar tissue located above and some below the soft palate (and to the back of the oral cavity). It is believed that Waldeyer's ring prevents the invasion of microorganisms from going into the air and food passages and this helps in the defense mechanism of the respiratory and alimentary systems. [ 7 ]


The word  pharynx  ( / ˈ f ær ɪ ŋ k s / [ 8 ] [ 9 ] ) is derived from the  Greek  φάρυγξ  phárynx , meaning "throat". Its plural form is  pharynges   / f ə ˈ r ɪ n dʒ iː z /  or  pharynxes   / ˈ f ær ɪ ŋ k s ə z / , and its adjective form is  pharyngeal  ( / ˌ f æ r ɪ n ˈ dʒ iː əl /   or   / f ə ˈ r ɪ n dʒ i əl / ).


All vertebrates have a pharynx, used in both feeding and respiration. The pharynx arises during development in all vertebrates through a series of six or more outpocketings on the lateral sides of the head. These outpocketings are  pharyngeal arches , and they give rise to a number of different structures in the skeletal, muscular, and circulatory systems. The structure of the pharynx varies across the vertebrates. It differs in dogs, horses, and ruminants. In dogs, a single duct connects the nasopharynx to the nasal cavity. The tonsils are a compact mass that points away from the lumen of the pharynx. In the horse, the auditory tube opens into the guttural pouch and the tonsils are diffuse and raised slightly. Horses are unable to breathe through the mouth as the free apex of the rostral epiglottis lies dorsal to the soft palate in a normal horse. In  ruminants , the tonsils are a compact mass that points towards the lumen of the pharynx.


Pharyngeal arches are characteristic features of vertebrates whose origin can be traced back through  chordates  to basal  deuterostomes  who also share endodermal outpocketings of the pharyngeal apparatus. Similar patterns of gene expression can be detected in the developing pharynx of  amphioxi  and  hemichordates . However, the vertebrate pharynx is unique in that it gives rise to  endoskeletal  support through the contribution of  neural crest  cells. [ 10 ]


Pharyngeal jaws  are a "second set" of  jaws  contained within the pharynx of many species of fish, distinct from the primary (oral) jaws. Pharyngeal jaws have been studied in  moray eels  where their specific action is noted. When the moray bites  prey , it first bites normally with its oral jaws, capturing the prey. Immediately thereafter, the pharyngeal jaws are brought forward and bite down on the prey to grip it; they then retract, pulling the prey down the eel's esophagus, allowing it to be swallowed. [ 11 ]


Invertebrates  also have a pharynx. Invertebrates with a pharynx include the  tardigrades , [ 12 ]   annelids  and  arthropods , [ 13 ]  and the  priapulids  (which have an eversible pharynx). [ 14 ]


The "pharynx" of the  nematode  worm is a muscular food pump in the head, triangular in cross-section, that grinds food and transports it directly to the intestines. A one-way valve connects the pharynx to the excretory canal.


This article incorporates text in the  public domain  from  page 1141  of the 20th edition of   Gray's Anatomy   (1918)


General


The  nervous system , and  endocrine system  collaborate in the  digestive system  to control gastric secretions, and motility associated with the movement of food throughout the  gastrointestinal tract , including  peristalsis , and  segmentation contractions .


Gastric activity involved in digestion is divided into three  phases of digestion  known as the  cephalic phase , the gastric phase, and the intestinal phase. These phases overlap and all three can occur simultaneously. [ 1 ]


A fourth phase of acid secretion is known as the basal state which occurs in the times between meals (interdigestive phase). The level of acid secretion during these times is regulated by body weight, individual, number of parietal cells, and time of day. Acid secretion is lowest in the morning before awakening and highest at night. [ 2 ]


The cephalic phase of digestion is the stage in which the stomach responds to the mere sight, smell, taste, or thought of food. About 20% of total acid secretion occurs before food enters the stomach. These sensory and mental inputs converge on the hypothalamus to induce the responses needed for preparing the  gastrointestinal tract  for food processing, which relays signals to the  medulla oblongata . [ 3 ]  Vagus nerve fibers from the medulla stimulate the parasympathetic nervous system of the stomach which, in turn, stimulates gastric secretion (via parietal and G cells). [ 1 ]  This enhanced secretory activity brought on by the thought or sight of food is a conditioned reflex. It only occurs when food is desired. When appetite is depressed this part of the cephalic reflex is inhibited.


Sensory stimuli from food activate dorsal motor nucleus of  vagus nerve  in the  medulla  (activating the  parasympathetic nervous system ). Insulin induced hypoglycemia also stimulates the vagus nerve. This results in four distinct physiological events.


1) In the body of the stomach, the vagal postganglionic muscarinic nerves release  acetylcholine  (ACh) which stimulates  parietal cell  H+ secretion.


2) In the lamina propria of the body of the stomach the ACh released from the vagal endings triggers histamine secretion from  ECL cells . Histamine also stimulates H+ secretion from parietal cells.


3) In the antrum, peptidergic postganglionic parasympathetic vagal neurons and other enteric nervous system neurons release GRP which stimulates antral  G cells  to produce and release  gastrin . Gastrin stimulates  gastric acid  secretion by directly stimulating parietal cells as well as by promoting histamine secretion by ECL cells.


4) In both the antrum and corpus, the vagus nerve inhibits  delta cells  (D cells), thus reducing their release of somatostatin and reducing background inhibition of gastrin release. [ 2 ]


Gastric chief cells  are primarily activated by ACh. However the decrease in pH caused by activation of parietal cells further activates gastric chief cells. Alternatively, acid in the  duodenum  can stimulate  S cells  to secrete  secretin  which acts on an endocrine path to deactivate gastric chief cells.


50-60% of total gastric acid secretion occurs during this phase. The gastric phase is a period in which swallowed food and semidigested protein (peptides and amino acids) activate gastric activity.   Ingested food stimulates gastric activity in two ways: by stretching the stomach and by gastric contents stimulating receptors in the stomach. [ 2 ]   Stretch activates two reflexes: a short reflex mediated through the myenteric nerve plexus, and a long reflex mediated through the vagus nerves and brainstem. [ 1 ]


1.)  Vagovagal reflex : Distention, or stretching, activates an afferent pathway which in turn stimulates efferent response from the dorsal nucleus of the vagus nerve. Stimulation of acid secretion occurs as it does in the cephalic phase.


2.) Local ENS Pathway: Activated ENS releases ACh stimulating parietal cells to secrete acid. [ 2 ]


As dietary protein is digested, it breaks down into smaller peptides and amino acids, which directly stimulate the G cells to secrete even more gastrin – a positive feedback loop that accelerates protein digestion. As discussed earlier gastrin stimulates by activating parietal cells and stimulating ECL to produce histamine (histamine stimulates parietal cells to produce acid). [ 2 ]  Small peptides also buffer stomach acid so the pH does not fall excessively low.


Gastric secretion is stimulated chiefly by three chemicals:  acetylcholine  (ACh),  histamine , and  gastrin .  ACh is secreted by  parasympathetic  nerve fibers of both the short and long reflex ,.ml; pathways.   Histamine  is a paracrine secretion from the enteroendocrine cells in the gastric glands.   Gastrin  is a hormone produced by enteroendocrine G cells in the pyloric glands. [ 1 ]


All three of these stimulate parietal cells to secrete  hydrochloric acid  and intrinsic factor.  The chief cells secrete  pepsinogen  in response to gastrin and especially Ach, and ACh also stimulates mucus secretion. [ 1 ]


Low intragastric pH stimulates antral D cells to release  somatostatin . Somatostatin inhibits gastrin release from G cells. Reduced gastrin secretion reduces acid secretion. [ 2 ]


5-10% of gastric secretion occurs during this phase. [ 2 ]


The intestinal phase is a stage in which the  duodenum  responds to arriving  chyme  and moderates gastric activity through hormones and nervous reflexes.  The  duodenum  initially enhances gastric secretion, but soon inhibits it.


The presence of partially digested proteins and amino acids in the duodenum stimulates acid secretion in the stomach by four methods:


1.) Peptones stimulate duodenal  G cells  to secrete gastrin.


2.) Peptones stimulate an unknown endocrine cell to release an additional humoral signal, "enterooxytonin".


3.) Amino acids absorbed by the duodenum stimulate acid secretion by unknown mechanisms.


4.) Osmolarity due to products of digestion stimulate acid secretion


The acid and semi-digested fats in the  duodenum  trigger the enterogastric reflex – the  duodenum  sends inhibitory signals to the stomach by way of the enteric nervous system, and sends signals to the medulla that (1) inhibit the vagal nuclei, thus reducing vagal stimulation of the stomach, and (2) stimulate sympathetic neurons, which send inhibitory signals to the stomach.  Chyme also stimulates duodenal enteroendocrine cells to release secretin and cholecystokinin.  They primarily stimulate the pancreas and gall bladder, but also suppress gastric secretion and motility.  The effect of this is that gastrin secretion declines and the pyloric sphincter contracts tightly to limit the admission of more chyme into the duodenum.  This gives the duodenum time to work on the chyme it has already received before being loaded with more. [ 1 ]
The enteroendocrine cells also secrete glucose dependent insulinotropic peptide.  Originally called gastric-inhibitory peptide, it is no longer thought to have a significant effect on the stomach, but to be more concerned with stimulating insulin secretion in preparation for processing the nutrients about to be absorbed by the small intestine. [ 1 ]


There is small continuous basal secretion of gastric acid between meals of usually less than 10 mEq/hour. [ 4 ]


The  basal electrical rhythm  controls the smooth muscle of the stomach and intestines, and controls the actions of  peristalsis , and  segmentation contractions .


The  polythyridium  is a component of the gut that surrounds the entrance of the intestine in  priapulids  of genus  Tubiluchus . It comprises tuberculate and hairy plates associated with a strong muscle. These plates may help to keep unwanted particles out of the gut. [ 1 ]


This  animal anatomy –related article is a  stub . You can help Wikipedia by  expanding it .


This  protostome -related article is a  stub . You can help Wikipedia by  expanding it .


Potential renal acid load  (PRAL) is a measure of the acid that the body produces after ingesting a food. This is different from pH, which is the acidity of a food before being consumed. [ 1 ] [ 2 ]  PRAL is a different acidity measure than the food ash measurement. [ 3 ]


Some acidic foods actually have a negative PRAL measurement, meaning they reduce acidity in the stomach. [ 4 ] [ 5 ]


A low PRAL diet (not to be confused with an  alkaline diet ) can lower acidity in the stomach, which can be helpful for people suffering  GERD  or Acid Reflux. However, it does not lower the pH of blood and therefore cannot treat osteoporosis or other conditions. [ 6 ]


This  medical  article is a  stub . You can help Wikipedia by  expanding it .


Prebiotics  are compounds in food that foster growth or activity of beneficial  microorganisms  such as bacteria and fungi. [ 1 ]  The most common environment concerning their effects on human health is the gastrointestinal tract, where prebiotics can alter the composition of organisms in the  gut   microbiome .


Dietary prebiotics are typically nondigestible  fiber  compounds that pass undigested through the upper part of the  gastrointestinal tract  and help growth or activity of advantageous bacteria in the  colon  by acting as  substrates  for them. [ 1 ]  They were first identified and named by Marcel Roberfroid in 1995. [ 1 ] [ 2 ]  Depending on the jurisdiction, they may have regulatory scrutiny as  food additives  for the  health claims  made for marketing purposes. Common prebiotics used in food manufacturing include  beta-glucan  from  oats ,  resistant starch  from grains and beans, and  inulin  from  chicory root .


The definition of prebiotics and the food ingredients that can fall under this classification, has evolved since its first definition in 1995. [ 3 ]  In its earliest definition, the term prebiotics was used to refer to non-digestible food ingredients that were beneficial to the host through their selective stimulation of specific bacteria within the  colon . [ 3 ] [ 4 ]  Further research has suggested that selective stimulation has not been scientifically demonstrated. [ 5 ]  As a result of research suggesting that prebiotics could impact microorganisms outside of the colon, in 2016 the  International Scientific Association for Probiotics and Prebiotics  (ISAPP) produced the following definition of prebiotics: a  substrate  that is selectively used by a host microorganism to produce a health benefit. [ 3 ]  In 2021, The Global Prebiotic Association (GPA) defined a prebiotic as a product or ingredient that is utilized in the microbiota producing a health or performance benefit. [ 6 ]


Compounds that can be classified as prebiotics must also meet the following criteria: [ 3 ] [ 4 ] [ 6 ]


Thus, consumption of prebiotics may facilitate the health of the host. [ 7 ]  Based on the previous classifications, plant-derived carbohydrate compounds called  oligosaccharides  as well as resistant starch are the main source of prebiotics that have been identified. [ 8 ] [ 4 ] [ 9 ] [ 10 ]  Specifically,  fructans  and  galactans  are two oligosaccharide sources which have been found to stimulate the activity and growth of beneficial bacterial colonies in the gut. [ 7 ] [ 3 ]  Fructans are a category of carbohydrate consisting of  fructooligosaccharides  (FOS) and  inulins , while galactans consist of  galactooligosaccharides . [ 3 ]  Resistant starch has been shown to shift the intestinal bacteria, as well as improve biomarkers for numerous health conditions. [ 11 ] [ 12 ] [ 13 ]   Other dietary fibers also fit the definition of prebiotics, such as  pectin , [ 14 ]   beta-glucans , [ 15 ]  and  xylooligosaccharides . [ 16 ]


The  European Food Safety Authority  (EFSA), the regulatory agency for product labeling, differentiates between "prebiotic" and "dietary fiber", stating that "a  cause and effect relationship  has not been established between the consumption of the food constituents which are the subject of the health claims and a beneficial physiological effect related to increasing numbers of gastrointestinal microbiota". [ 17 ]  Consequently, under EFSA rules individual ingredients cannot be labeled as prebiotics, but only as dietary fiber and with no implication of health benefits. [ 17 ]


When the prebiotic concept was first introduced in 1995, the primary focus was on the effects that prebiotics confer on  Bifidobacteria  and  Lactobacillus . [ 3 ] [ 4 ] [ 18 ]  With improved mechanistic techniques in recent years, the current prebiotic targets have expanded to a wider range of microbes, including  Roseburia  spp.,  Eubacterium  spp.,  Akkermansia  spp.,  Christensenella  spp.,  Propionibacterium  spp. and  Faecalibacterium  spp. [ 19 ]  These bacteria have been highlighted as key  probiotics  and beneficial gut bacteria as they may have several beneficial effects on the host in terms of improving digestion (including but not limited to enhancing mineral absorption) [ 20 ]  and the effectiveness and intrinsic strength of the immune system. [ 21 ]  Both  Bifidobacteria  and  Lactobacillus  have been shown to have differing prebiotic specificity and to selectively ferment prebiotic fiber based on the enzymes characteristic of the bacterial population. [ 22 ]  Thus,  Lactobacilli  prefer inulin and fructooligosaccharides, while  Bifidobacteria  display specificity for inulin, fructooligosaccharides, xylooligosaccharides and galactooligosaccharides. [ 22 ]   Studies have also shown that prebiotics, besides helping growth of beneficial gut bacteria, can also inhibit detrimental and potentially pathogenic microbes in the gut, [ 9 ] [ 4 ]  such as  clostridia . [ 4 ]


Fermentation  is the main  mechanism of action  by which prebiotics are used by beneficial bacteria in the colon. [ 7 ] [ 4 ]  Both  Bifidobacteria  and  Lactobacillus  are bacterial populations which use  saccharolytic  metabolism to break down substrates. [ 4 ]  The bifidobacterial  genome  contains many  genes  that encode for carbohydrate-modifying enzymes as well as genes that encode for carbohydrate uptake proteins. The presence of these genes indicates that  Bifidobacteria  contain specific metabolic pathways specialized for the fermentation and metabolism of plant-derived oligosaccharides, or prebiotics. These pathways in  Bifidobacteria  ultimately produce  short chain fatty acids , [ 4 ] [ 7 ]  which have diverse  physiological  roles in body functions. [ 23 ] [ 3 ]


Prebiotic sources must be proven to confer a benefit to the host in order to be classified as a prebiotic. [ 3 ]  Fermentable  carbohydrates  derived from  fructans  and  xylans  are one well documented example of prebiotics. [ 3 ]  Resistant starch from starchy foods are also well documented prebiotics and have historically been the highest source of prebiotics in the diet, as 4-10% of starch in mixed diets has been shown to reach the large intestine. [ 24 ]  One study reported that individuals consuming a traditional diet in Africa consumed 38 grams of resistant starch/day. [ 25 ]


An endogenous source of prebiotics in humans is human  breast milk , which contains  oligosaccharides  structurally similar to  galactooligosaccharides , referred to as human milk oligosaccharides. [ 26 ] [ 9 ] [ 22 ] [ 3 ]  Human milk oligosaccharides were found to increase the  Bifidobacteria  bacterial population in breastfed infants, and to strengthen the infant immune system. [ 3 ] [ 9 ]  Furthermore, human milk oligosaccharides help establish a healthy intestinal microbiota composition in newborns. [ 3 ]


Indigestible carbohydrate compounds classified as prebiotics are a type of  fermentable  fiber, and thus can be classified as  dietary fiber . [ 4 ]  However, not all dietary fiber can be classified as a prebiotic source. [ 4 ]  In addition to the food sources highlighted in the following table, raw oats, [ 18 ]   unrefined barley , [ 18 ]   yacón , [ 18 ]  and whole grain  breakfast cereals [ 4 ]  are also classified as prebiotic fiber sources. The predominant type of prebiotic fiber may vary according to the food. For instance, oats and barley have high amounts of  beta-glucans , fruit and berries contain  pectins , seeds contain  gums , onions and  Jerusalem artichokes  are rich in  inulin  and  oligofructose , and bananas and legumes contain  resistant starch . [ 27 ]


While there is no broad consensus on an ideal daily serving of prebiotics, recommendations typically range from 4 to 8 grams (0.14–0.28 oz) for general digestive health support, to 15 grams (0.53 oz) or more for those with active digestive disorders. Given an average 6 grams (0.21 oz) serving, below are the amounts of prebiotic foods required to achieve a daily serving of prebiotic fiber:


Preliminary research has demonstrated potential effects on calcium and other mineral absorption, [ 29 ]   immune system  effectiveness, [ 30 ] [ 31 ]   bowel  acidity, reduction of  colorectal cancer  risk, [ 32 ]   inflammatory bowel disease  ( Crohn's disease  or  ulcerative colitis ), [ 33 ]   hypertension [ 34 ]  and  defecation  frequency. [ 35 ]  Prebiotics may be effective in decreasing the number of infectious episodes needing antibiotics and the total number of infections in children aged 0–24 months. [ 31 ] [ 36 ]


No good evidence shows that prebiotics are effective in preventing or treating allergies. [ 37 ]


While research demonstrates that prebiotics lead to increased production of  short-chain fatty acids  (SCFA), [ 38 ]  more research is required to establish a direct causal connection. Prebiotics may be beneficial to  inflammatory bowel disease  or Crohn's disease through production of SCFA as nourishment for colonic walls, and mitigation of ulcerative colitis symptoms. [ 39 ]


The sudden addition of substantial quantities of prebiotics to the diet may result in an increase in  fermentation , leading to increased gas production,  bloating  or  bowel movement . [ 40 ]  Production of SCFA and fermentation quality are reduced during long-term diets of low fiber intake. [ 41 ]  Until bacterial flora are gradually established to rehabilitate or restore intestinal bacteria, nutrient absorption may be impaired and colonic transit time temporarily increased with a rapid addition of higher prebiotic intake. [ 40 ] [ 42 ]


Genetically modified plants  have been created in research labs with upregulated inulin production. [ 43 ] [ 44 ]


Probiotics  are live  microorganisms  promoted with claims that they provide health benefits when consumed, generally by improving or restoring the  gut microbiota . [ 1 ] [ 2 ]  Probiotics are considered  generally safe to consume , but may cause  bacteria - host  interactions and unwanted  side effects  in rare cases. [ 3 ] [ 4 ] [ 5 ]  There is some evidence that probiotics are beneficial for some conditions, such as helping to ease some symptoms of irritable bowel syndrome (IBS). However, many claimed health benefits, such as treating eczema, lack substantial scientific support. [ 1 ]


The first discovered probiotic was a certain strain of  bacillus  in Bulgarian yoghurt, called  Lactobacillus bulgaricus . The discovery was made in 1905 by Bulgarian physician and microbiologist  Stamen Grigorov . The modern-day theory is generally attributed to Russian  Nobel Prize  laureate  Élie Metchnikoff , who postulated around 1907 that  yoghurt -consuming Bulgarian peasants lived longer. [ 6 ]


A growing probiotics market has led to the need for stricter requirements for  scientific substantiation  of putative benefits conferred by microorganisms claimed to be probiotic. [ 7 ]  Although some evidence claimed benefits are marketed towards using probiotic, such as reducing  gastrointestinal  discomfort, improving  immune health , [ 8 ]  relieving  constipation , or avoiding the  common cold , such claims are strain-specific and cannot be extrapolated to other strains. [ 7 ] [ 9 ] [ 10 ]  As of 2019, numerous applications for approval of  health claims  by European manufacturers of probiotic  dietary supplements  have been rejected by the  European Food Safety Authority  for insufficient evidence of beneficial mechanism or efficacy. [ 8 ] [ 11 ]