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The earliest amniotes, including stem-reptiles those amniotes closer to modern reptiles than to mammals , were largely overshadowed by larger stem-tetrapods, such as Cochleosaurus , and remained a small, inconspicuous part of the fauna until the Carboniferous Rainforest Collapse. Chemical action includes the release of digestive enzymes and fluids from various parts of the digestive system. Ruminants cattle, sheep, and goats lack which of the following teeth? In artificial incubation and brooding, chicks do not have this option. Birds use their momentum to travel underwater at an average descent rate of 2.
The large intestine also called the colon, consists of the cecum , rectum , and anal canal. It also includes the appendix , which is attached to the cecum. The colon is further divided into:.
The main function of the large intestine is to absorb water. The area of the large intestinal mucosa of an adult human is about 2 m 2. The gut is an endoderm -derived structure. At approximately the sixteenth day of human development, the embryo begins to fold ventrally with the embryo's ventral surface becoming concave in two directions: The result is that a piece of the yolk sac , an endoderm -lined structure in contact with the ventral aspect of the embryo, begins to be pinched off to become the primitive gut.
The yolk sac remains connected to the gut tube via the vitelline duct. Usually this structure regresses during development; in cases where it does not, it is known as Meckel's diverticulum.
During fetal life, the primitive gut is gradually patterned into three segments: Although these terms are often used in reference to segments of the primitive gut, they are also used regularly to describe regions of the definitive gut as well.
Each segment of the gut is further specified and gives rise to specific gut and gut-related structures in later development. Components derived from the gut proper, including the stomach and colon , develop as swellings or dilatations in the cells of the primitive gut. In contrast, gut-related derivatives — that is, those structures that derive from the primitive gut but are not part of the gut proper, in general develop as out-pouchings of the primitive gut.
The blood vessels supplying these structures remain constant throughout development. The gastrointestinal tract has a form of general histology with some differences that reflect the specialization in functional anatomy. The mucosa is the innermost layer of the gastrointestinal tract. The mucosa surrounds the lumen , or open space within the tube. This layer comes in direct contact with digested food chyme.
The mucosa is made up of:. The mucosae are highly specialized in each organ of the gastrointestinal tract to deal with the different conditions. The most variation is seen in the epithelium. The submucosa consists of a dense irregular layer of connective tissue with large blood vessels, lymphatics, and nerves branching into the mucosa and muscularis externa. It contains the submucosal plexus , an enteric nervous plexus , situated on the inner surface of the muscularis externa. The muscular layer consists of an inner circular layer and a longitudinal outer layer.
The circular layer prevents food from traveling backward and the longitudinal layer shortens the tract. The layers are not truly longitudinal or circular, rather the layers of muscle are helical with different pitches. The inner circular is helical with a steep pitch and the outer longitudinal is helical with a much shallower pitch. Whilst the muscularis externa is similar throughout the entire gastrointestinal tract, an exception is the stomach which has an additional inner oblique muscular layer to aid with grinding and mixing of food.
The muscularis externa of the stomach is composed of the inner oblique layer, middle circular layer and outer longitudinal layer. Between the circular and longitudinal muscle layers is the myenteric plexus. Activity is initiated by the pacemaker cells, myenteric interstitial cells of Cajal. The gut has intrinsic peristaltic activity basal electrical rhythm due to its self-contained enteric nervous system.
The rate can be modulated by the rest of the autonomic nervous system. The coordinated contractions of these layers is called peristalsis and propels the food through the tract.
Food in the GI tract is called a bolus ball of food from the mouth down to the stomach. After the stomach, the food is partially digested and semi-liquid, and is referred to as chyme. In the large intestine the remaining semi-solid substance is referred to as faeces. The outermost layer of the gastrointestinal tract consists of several layers of connective tissue. Intraperitoneal parts of the GI tract are covered with serosa. These include most of the stomach , first part of the duodenum , all of the small intestine , caecum and appendix , transverse colon , sigmoid colon and rectum.
In these sections of the gut there is clear boundary between the gut and the surrounding tissue. These parts of the tract have a mesentery. Retroperitoneal parts are covered with adventitia.
They blend into the surrounding tissue and are fixed in position. For example, the retroperitoneal section of the duodenum usually passes through the transpyloric plane. These include the esophagus , pylorus of the stomach, distal duodenum , ascending colon , descending colon and anal canal. In addition, the oral cavity has adventitia. Specific proteins expressed in the stomach and duodenum involved in defence include mucin proteins, such as mucin 6 and intelectin Finally, transit through the colon takes 12 to 50 hours with wide variation between individuals.
The gastrointestinal tract forms an important part of the immune system. There are additional factors contributing to protection from pathogen invasion. For example, low pH ranging from 1 to 4 of the stomach is fatal for many microorganisms that enter it. Beneficial bacteria also can contribute to the homeostasis of the gastrointestinal immune system. For example Clostridia , one of the most predominant bacterial groups in the GI tract, play an important role in influencing the dynamics of the gut's immune system.
This is due to the production of short-chain fatty acids during the fermentation of plant-derived nutrients such as butyrate and propionate. Basically, the butyrate induces the differentiation of Treg cells by enhancing histone H3 acetylation in the promoter and conserved non-coding sequence regions of the FOXP3 locus, thus regulating the T cells , resulting in the reduction of the inflammatory response and allergies.
The large intestine hosts several kinds of bacteria that can deal with molecules that the human body cannot otherwise break down. These bacteria also account for the production of gases at host-pathogen interface , inside our intestine this gas is released as flatulence when eliminated through the anus.
However the large intestine is mainly concerned with the absorption of water from digested material which is regulated by the hypothalamus and the re absorption of sodium , as well as any nutrients that may have escaped primary digestion in the ileum.
Health-enhancing intestinal bacteria of the gut flora serve to prevent the overgrowth of potentially harmful bacteria in the gut. These two types of bacteria compete for space and "food," as there are limited resources within the intestinal tract.
Enzymes such as CYP3A4 , along with the antiporter activities, are also instrumental in the intestine's role of drug metabolism in the detoxification of antigens and xenobiotics. Embryonated ova and free larval forms may be seen in the feces. Larvae resembling Rhabdias also have been seen in the gingiva of snakes with stomatitis. Infections often are subclinical but may be associated with secondary bacterial pneumonia. In severe cases, death may result. Stomach worms of the genus Physaloptera are seen in lizards.
Gastric ulceration may occur in severe infections. Ova are elliptical and may be embryonated. Numerous snakes are infected by Kalicephalus spp. This hookworm, capable of transcutaneous infestation, prefers the upper GI tract and causes erosive lesions at sites of attachment. Ova are similar to those of Physaloptera spp. Large granulomas caused by the above species have also caused GI obstruction in snakes. Ascarids frequently infect reptiles. Ova are similar to those of ascarids from mammalian hosts.
Severe lesions and death may be seen in infected snakes. Clinically infected snakes frequently regurgitate partially digested food or adult nematodes and are anorectic. The major lesions are large granulomatous masses in the GI tract; they may abscess and perforate the intestinal wall. Many other nematode species may be found in reptiles. Capillarid, trichurid, and oxyurid ova may be found on fecal examination.
The nonpathogenic larval and oval forms of parasites of prey items eg, Syphacia obvelata , the mouse pinworm may be found when infected prey is consumed. Treatment should be attempted when evidence of parasitism is present. Some larval forms of nematodes are suspected or confirmed to penetrate the skin eg, Strongyloides and Kalicephalus , bypassing the oral reinfection route.
The subtle nature of reinfection by this route often goes unnoticed until the reptile is overwhelmed by parasites. Close attention to the immediate removal of excreta and fastidious sanitation help reduce parasite burdens in captivity.
Dermal lesions caused by the spirurid worm Dracunculus spp may be seen. Numerous species of spirurids infect the mesentery, coelomic cavity, and blood vessels. These worms require a mechanical vector, so their incidence is reduced in captive-bred reptiles or in reptiles that have been in captivity longterm.
Pentastomes are found in a wide variety of reptiles, with variable pathogenicity. Pentastomid infections are occasionally associated with pneumonic signs, but these primitive arthropods can inhabit any tissue, and symptoms will vary with their migration path and tissues responses. Pentastomes were initially found primarily in tropical poisonous snakes; however, as more necropsies on reptiles were performed, more were found. Necropsy results from 88 bearded dragons showed that 11 were infested with pentastomes.
The most novel approach has been to endoscopically locate and mechanically remove all the adult pentastomes. Recognition of pentastomal infestations is important, because these parasites are thought to present a zoonotic risk. Numerous protozoans are found on reptiles; most are harmless commensals. The most serious protozoal pathogen of reptiles is Entamoeba invadens. Clinical signs are anorexia, weight loss, vomiting, mucoidal or hemorrhagic diarrhea, and death.
Entamoebiasis may be epidemic in large snake collections. Herbivores appear less susceptible than carnivores; a number of reptiles that seldom become affected or die can serve as carriers, including garter snakes, northern black racers, and box turtles. Although most turtles are resistant, the giant tortoises are susceptible. Other resistant groups include eastern king snakes, crocodiles, and cobras possibly as an adaptation that allows them to eat snakes.
Most boas, colubrids, elapids, vipers, and crotalids are highly susceptible. Transmission is by direct contact with the cyst form. Hepatic abscesses containing numerous E invadens trophozoites are common in chronic cases. The small intestine is made up of the duodenum also referred to as the duodenal loop and the lower small intestine. The remainder of the digestion occurs in the duodenum, and the released nutrients are absorbed mainly in the lower small intestine.
The duodenum receives digestive enzymes and bicarbonate to counter the hydrochloric acid from the proventriculus from the pancreas and bile from the liver via the gall bladder. The lower small intestine is composed of two parts, the jejunum and the ileum.
The Meckel's diverticulum marks the end of the jejunum and the start of the ileum see Figure 6. In the egg, the yolk sac supplies the nutrients needed for the embryo to develop and grow. Right before hatch, the yolk sac is taken into the navel cavity of the embryo.
The residual tiny sac is the Meckel's diverticulum. Location of the Meckel's diverticulum in the digestive tract of a chicken. The ceca plural form of cecum are two blind pouches located where the small and large intestines join. Some of the water remaining in the digested material is reabsorbed here. Another important function of the ceca is the fermentation of any remaining coarse materials.
During this fermentation, the ceca produce several fatty acids as well as the eight B vitamins thiamine, riboflavin, niacin, pantothenic acid, pyridoxine, biotin, folic acid, and vitamin B Because the ceca are located so close to the end of the digestive tract, however, few of the produced nutrients are absorbed and available to the chicken.
Despite the name, the large intestine is actually shorter than the small intestine. The large intestine is where the last of the water reabsorption occurs.
In the cloaca, the digestive wastes mix with wastes from the urinary system urates. Chickens usually void fecal material as digestive waste with uric acid crystals on the outer surface—that is, chickens do not urinate. The color and texture of chicken fecal material can indicate the health status of the chicken's digestive tract: The reproductive tract also exits through this area.
When a hen lays an egg, the vagina folds over to allow the egg to leave through the cloaca opening without coming into contact with feces or urine. These microflora aid in digestion.
When chicks hatch, their digestive tracts are virtually sterile. If raised by a mother hen, a chick obtains the beneficial microflora by consuming some of its mother's fecal material. In artificial incubation and brooding, chicks do not have this option. Through the probiotics, the chicks receive the beneficial bacteria they need to fight off infection by pathogenic bacteria, such as salmonella.
Intestinal disease in chickens normally occurs when the balance of normal microflora is upset—that is, the normal microflora are overrun by too many foreign organisms. The result is enteritis, or inflammation of the intestines. Enteritis produces symptoms that include diarrhea, increased thirst, dehydration, loss of appetite, weakness, and weight loss or slow growth.