< back to Home page CD Table of Contents References
THE DIGESTIVE SYSTEM OF
VERTEBRATES: CD 
5. Anatomy of the Digestive Tract:
Fish:
The anatomy of the fish digestive system has been reviewed by Harder (1975a,b) and Stevens and Hume (1995). Major variations in the headgut, pharynx, foregut and upper midgut are illustrated in Figure 5.1. The headgut and foregut show a considerable degree of species variation. Articulated jaws are absent in the cyclostomes, which are parasitic on other fish, but present in all other fish and advanced vertebrates. Teeth vary in both their location (jaw, tongue, pharynx or other surfaces of the orobranchial cavity) and function. The teeth of most fish are used for grasping, cutting, or tearing, and loss of food through the gills is limited by gill rakers in many species. However, some species such as the chub, grass carp, and parrotfish use pharyngeal teeth to grind food to a small particle size, and the basking sharks, whale sharks, and paddlefish are microphagus filter-feeders. The esophagus of fish is relatively short and the stomach is absent in cyclostomes and some of the more advanced groups with pharyngeal teeth. Where present, the stomach is straight, U-shaped, or Y-shaped with a blind sac on its greater curvature.
The midgut of fish can vary from one that is short and straight to a longer structure with spirals and loops. In some species with a short midgut, its mucosal surface and digesta retention time are increased by folds, which form a spiral valve, or the presence of anywhere from 1 to 1000 pyloric ceca (Fig. 5.1). Pancreatic tissue is found in midgut ceca of cyclostomes and distributed along the midgut of many of the more advanced species of fish, but the pancreas is a compact organ in some cartilaginous and teleost fish, and all other vertebrates. The biliary secretions of the liver are stored in a gallbladder by most fish, but a gallbladder is absent in some species and both the gallbladder and hepatic ducts disappear following metamorphosis of lampreys to the adult stage of feeding.
Figure 5.1. Digestive tracts of fish: sea lamprey (Petromyzon marinus), chub (Leuciscus cephalus), pike (Esox lucius), trout (Salmo fario), eel (Anguilla anguilla). Articulated jaws are absent in cyclostomes, such as the lamprey, and located in the pharynx (1) of some species such as the chub. The esophagus (2) varies in length and the stomach is absent in cyclostomes and some advanced species, such as the chub. Where present, the stomach (3) may be straight (pike), U-shaped (trout), or Y-shaped with a gastric cecum (eel). The absorptive surface and digesta retention time of the midgut (4) is increased by a spiral valve (5) or pyloric ceca (6) in a number of species. (From Harder 1975a.)
The hindgut of most fish is short and difficult to distinguish from the midgut with respect to changes in diameter or epithelial morphology. However, an ileorectal valve is present in many teleosts, and a small cecum is found in some catfish, and the knifefish, cod, and sea chubs. The relative rarity of herbivorous fish has been attributed to limitations in the masticatory apparatus and gut capacity of most species. The herbivores can be classified into four categories, based on adaptations designed to degrade cell walls of plants by acid lysis, mechanical trituration, or microbial fermentation (Fig. 5.2). The first group, which includes surgeonfish, consists of species with no mechanisms for the trituration of food and a thin-walled stomach, but relatively long midgut. The second group, which includes the mullets, features a thick walled, gizzard-like segment of stomach and a midgut of variable length. The third group includes the parrotfish, scarids, odacids, and herbivorous carp, which have pharyngeal teeth, no stomach, and a relatively short intestine. Group four, which is seen in the sea chubs, has a long intestine that includes a distinct hindgut with sphincters at its junctures with the midgut and rectum, and a pair of small ceca.
Figure 5.2. Digestive strategies of herbivorous marine fish. Surgeonfish and parrotfish are browsers. Mullet and sea chub are grazers. Shaded areas indicate the gizzard-like stomach of the mullet, pyloric ceca of surgeonfish and sea bass, and two regions of sphincters in distal intestine the sea chub (Modified from Horn 1989.)
Amphibians:
Structural and functional variations of the amphibian digestive system are discussed by Reeder (1964) and Houdry, et al (1996). Some larval amphibians can engulf their entire prey of crustaceans, mosquito larva, or worms. Others have horny teeth for removal of encrusted material from plants or rocks, or complex, microphagous, filtering mechanisms for ingestion of bacteria, zooplankton, or phytoplankton. Most larval anurans (frogs and toads) lack a stomach. The gastric region of the digestive tract usually forms a thickened sheath, which produces mucus, a proteolytic cathepsin, and a low pH, but pepsin has been rarely reported. The intestine is relatively long, with no distinct separation into a midgut and hindgut. Although the brush border of its absorptive epithelial cells contains many of the digestive enzymes found in other vertebrates, there is evidence of phagocytosis and intracellular digestion in some species.
Amphibians undergo profound changes in their diet, feeding practices, and the structure and function of their digestive tract during the metamorphosis from larval to juvenile stages (McAvoy and Dixon 1977; Houdry et al 1996). Adult amphibians are carnivores with a weak dentition that serves for the grasping of food while it is being swallowed. A distensible tongue is used for the capture of prey by some species, and the mouth contains multicellular glands that secrete mucous. Esophageal glands that secrete pepsinogen have been described in frogs and toads, and those of the red-legged pan frog (Kassina maculata) are said to secrete more than the gastric glands (Hirja 1982). Metamorphosis is also accompanied by a considerable shortening of the intestine, removal and regeneration of intestinal epithelium, and the appearance of a distinct hindgut that is lined with columnar epithelium and goblet cells (Fig. 5.3).
Figure 5.3. Gastrointestinal tract of two adult amphibians, American toad, and tiger salamander. Body length in this and the similar drawings of other species represents distance from the most anterior region of the mouth to the anus. (From Stevens & Hume 1995.)
Reptiles:
The anatomy of the reptilian digestive system is described by Parsons and Cameron (1977), and Luppa (1977). The mouth parts of most reptiles are used for grasping, cutting, or tearing their food. This is accomplished with a beak in the chelonians and teeth in other reptiles. The jaws of snakes are arranged for distention and even disarticulation during ingestion of prey, and the fang teeth of some species are used to inject toxins or digestive enzymes. The teeth of mollusk-eating lizards are modified for crushing and herbivores such as the iguana have cusp-like teeth, but the upper and lower jaw of reptiles are of equal width and their articulation provides a scissors-like closure unsuitable for grinding of food into small particles (Ostrom 1963). Some species have a distensible tongue that serves as a sensing organ. The oral cavity of reptiles contains mucus secreting cells, and many species have complex oral glands, which secrete venoms and digestive enzymes in some snakes and lizards. However salivary glands are usually absent and, where present, secrete only mucus.
The gastrointestinal tracts of a carnivorous caiman, an omnivorous turtle, and a herbivorous tortoise and lizard are illustrated in Figure 5.4. The stomach of reptiles tends to be tubular, but it is larger and more outpocketed in crocodilians, with a muscular pylorus that is separated from the remainder of the stomach by a constriction. Gastroliths (stones, gravel or sand) have been reported in the stomach of chelonians, lizards, and a 100% of Crocodylus nycloticus over 2 m in length (Corbet 1960). The herbivorous Florida red-bellied turtle has an extremely long midgut and short hindgut, but the midgut is generally shortest in herbivores and longest in carnivores. The hindgut of most herbivores is longer that of other reptiles and it includes a blind sac or cecum at its juncture with the midgut. The cecum and proximal colon of herbivorous lizards in the families Agamidae, Scincidae, and Iguanidae are compartmentalized by mucosal folds, which slow digesta passage and increase the absorptive surface area.
Figure 5.4. Gastrointestinal tracts of a carnivorous caiman, snake, and forest chameleon, an omnivorous turtle, and a herbivorous tortoise and lizard (iguana). Note the cecum, larger volume, and greater relative length of the herbivore hindgut, and the baffles provided by projections of tissue into the cecum and colon of the iguana. (From Stevens & Hume 1995.)
Birds:
The structural characteristics of the avian digestive system are described by Ziswieler and Farner (1972) and Duke (1986). Modifications for flight have resulted in an absence of teeth, reduction in the weight of the jaw skeleton and muscles, and the acquisition of a gizzard as the organ for trituration. The avian bill or beak can serve for cutting, tearing, crushing, or other purposes, such as filter feeding in flamingos, but the jaw articulation is not constructed for efficient trituration or grinding of food. Salivary glands are usually present and highly developed. They function principally for mucigenous lubrication, but also secrete an adhesive substance in some species and amylase in others. The gastric functions of birds are carried out by a crop (storage), proventriculus (pepsinogen and HCl secretion) and gizzard (trituration). The proventriculus is lined with proper gastric and pyloric glandular mucosa. The gizzard is lined with kaolin, a horny material consisting of protein and carbohydrates, which is periodically molted by some species.
Figures 5.5 and 5.6 show the gastrointestinal tracts of a carnivorous, two omnivorous, and five herbivorous species. The relative size of the crop, proventriculus, and gizzard tends to vary with the diet. The crop tends to be smaller in carnivores, such as the hawk, and is absent in the herbivorous ostrich, but granivores such as the chicken and herbivores such as the ruffed grouse generally have a large crop and a large, muscular gizzard. The gizzard is smaller and less muscular in carnivores and species that feed principally on nectar, fruit, or pollen. Most birds have a relatively short midgut and a hindgut that consists of a short, straight colon and paired ceca. However, Poppema (1990) found that ceca were absent or poorly developed in all species belonging to 13 of the avian orders, including the small passerine species and most larger species that feed on carrion, nectar, fruit, or small vertebrates. The most well-developed ceca and highest ratio of cecal length/ total intestinal length were found in birds that fed on high levels of plant fiber or invertebrates. The ceca serve as a major site for microbial fermentation of plant fiber in most herbivores and, possibly, chitin in birds that feed on invertebrates.
Figure 5.5. Gastrointestinal tracts of a hawk, budgerigar, and chicken. The hawk drawing also shows the lumen surface of the crop, proventriculus and gizzard. Ceca are small in most carnivores, such as the hawk, and absent in some species, such as the budgerigar, but highly developed in the chicken (From Stevens & Hume 1995.)
Figure 5.6. Gastrointestinal tracts of avian herbivores. The crop is absent in the ostrich, but expanded in the grouse and rhea, and both the crop and distal esophagus are expanded in the hoatzin. Note the well-developed ceca in the grouse and rhea, the long small intestine of the emu, and extremely long colon of the ostrich. (From Stevens & Hume 1995.)
Avian herbivores adopted four different sites for the retention and microbial fermentation of plant material (Fig. 5.6). The principal site for microbial fermentation in grouse, partridge and rheas is the ceca, which are extremely large in rheas and increase in length during winter months in spruce grouse (Pendergast and Boag 1973) and rock ptarmigan (Gasaway et al. 1976a). However, the principal sites in the hoatzin, emu, and ostrich are an enlarged crop and distal esophagus, the midgut, and the colon, respectively. The long colon is a feature that appears to be unique to ostriches and horned screamers Aakima cornuta (Mitchell 1901).
Mammals - Headgut:
The anatomy of the mammalian digestive tract was reviewed by Stevens and Hume (1995). One of the major advances in the evolution of mammals was the acquisition of an extremely efficient masticatory apparatus (Crompton and Parker 1978). A few species of edentates have lost their teeth and the teeth of baleen whales are replaced with ridges of palatal mucous membranes that serve as a sieve for filter-feeding. Species in five mammalian orders (echidna, aardvarks, scaly anteaters, edentate anteaters, aardwolves) demonstrate convergence on weak jaws, relatively simple teeth, and a long tongue that are adapted for feeding exclusively on ants or termites. However, the teeth of most mammals include incisors for cutting, canines or fang teeth for grasping and tearing, and large premolar and molars with uneven occluding surfaces.
Mammalian anteaters, which have weak jaws, relatively simple teeth and a long tongue, are adapted for feeding exclusively on ants or termites (photos: aardvark and scaly anteater by Dr Michael Stoskopf, echidna and giant anteater by Dr Kerri Slifka, aardwolf by J. Visser)
Muscles in the cheeks and a mobile tongue aid in the positioning of food between the crushing and shearing surfaces of the premolars and molars. The articulation of the jaws and a complex musculature sling allows both a vertical movement of the lower jaw (mandible) and either its lateral movement, as seen in most mammals (Fig. 5.7a), or the anterior-posterior action seen in rodents and elephants. This provides the molars with an additional grinding function for the reduction of food to a small particle size. Much of the success of rodents has been attributed to their flexible masticatory apparatus, which allows the occlusion of the incisors for seizure of prey, clipping of leafs or stems, or removal of bark from shrubs and trees. However, the teeth of different mammalian species can vary in number, size, and construction (Fig. 5.7b).
Mammals have three pairs of salivary glands that can differ in both the volume and composition of their secretions (Ellison 1967; Leeson 1967; Phillipson 1970; Cook et al. 1994). The parotid glands usually secrete a serous fluid, and secretions of the submaxillary (submandibular) and sublingual glands tend to contain large amounts of mucus. The parotids are the largest glands in many herbivores, such as the artiodactyls, perissodactyls, macropod marsupials, manatees, and beavers, but the submaxillary gland of the giant anteater is extremely large and provided with a storage bladder.
Figure 5.7a. A longitudinal and cross section of the horse skull. Most mammals have teeth and jaws that aid in the procurement and breakdown of food. The lower jaw or mandible is narrower than the upper jaw, and its lateral to and fro movements provide an extremely efficient mechanism for grinding of food. (From Norman & Weishampel, 1985)
Figure 5.7b Skull of a South American Margay (Felis tigerina) and the mandibular tooth of an Asian elephant. The elephant tooth has a weight about equal to a telephone book, and the dark band (gingival crest) marks the separation between the root and the crown. (Contributed by David A. Fagan, The Colyer Institute, P. O. Box 26118, San Diego, CA)
Mammals - Foregut:
Although the stomach of most mammals is a relatively simple expansion of the digestive tract that is lined with cardiac, proper gastric, and pyloric mucosa, it can vary in its epithelial lining, size, and shape. The stomachs of some of the species in half of the mammalian orders contain an additional region of non-glandular, stratified squamous epithelium (Fig. 5.8). Stratified squamous epithelium occupies a small region of the stomach of the domesticated pig and the colobus and langur monkeys, a larger percentage of the stomach of scaly anteaters, cetaceans, macropod marsupials, sloths, perissodactyls, most artiodactyls, and many rodents, and the entire stomach of monotremes and armadillos. Cardiac glandular mucosa also varies from the narrow region seen in most species to the much wider regions witnessed in pigs and camelids.
Figure 5.8. Examples of mammal stomachs that contain a region of stratified squamous epithelium; echidna, scaly anteater, dolphin, kangaroo, armadillo, sloth, colobus monkey, rat, horse, and hyrax. (Modified from Stevens & Hume 1995)
The stomach of cetaceans, macropod marsupials, sirenians, hyrax, most artiodactyls, and some rodents and edentates includes an expanded segment of sacculated or compartmentalized forestomach (Fig. 5.9). Therefore, the terms cardiac, body, and fundus that are used to describe segments of the human stomach are useless for comparisons with many other species.
Figure 5.9. Examples of mammals with an expanded forestomach; hippopotamus, llama, kangaroo, ox, and sloth. E designates esophageal entrance, P designates pylorus, 1 designates omasum, and 2 designates abomasum. (Modified from Stevens & Hume 1995.)
Mammals - Hindgut:
The hindgut of mammals also varies from a simple structure with no cecum or valvular separation from the midgut to a voluminous, sacculated, multicompartmental organ. In most species it tends to be longer than that of other vertebrates, consisting of a colon, rectum, and often a cecum that is paired in a very few species. In some species, the cecum and varying lengths of the colon are drawn into sacculations (haustra) by the contraction of longitudinal bands of muscle in a fashion similar to that seen in the forestomach of kangaroos and colobid monkeys.
The colon of humans can be subdivided into ascending, transverse, and descending segments according to the direction it takes in the abdominal cavity. The mesenteric attachment of the transverse colon and the loop that it forms around the mesenteric artery allows a comparison with similar segments in other species (Fig. 5.10). The "ascending" colon is the segment of hindgut that has lengthened in most mammals. However, it can vary considerably in its length, volume, and course of direction. Therefore, these anatomical terms assigned to the human colon have little relevance for comparison to most other species.
Figure 5.10. The large intestine of the human, dog, horse, pig, and ox. Note that the cecum and segments homologous to the ascending, transverse and descending colon of humans vary in their relative length, shape, and volume, and that the proximal or "ascending" segment is extended and expanded in many mammals. (Modified from de Lahunta and Habel 1986.)
Mammals - Carnivores:
Examples of the gastrointestinal tracts of carnivorous species in the orders Monotremata, Cetacea, Macroscelidea, Insectivora, Chiroptera, Carnivora, and Marsupialia are illustrated in Figures 5.11, 5.12, and 5.13. The stomach of most carnivores consists of a unilateral dilatation of the digestive tract. However, cetaceans (porpoises, dolphins, whales) have a large multicompartmental stomach, which is believed to have been conserved from herbivorous ancestors closely related to the artiodactyls, and the stomach of vampire bats is convoluted and approximately twice the length of their body. The hindgut is very short and indistinct in some Insectivora, cetaceans, and marsupials, and it lacks a valvular separation from the midgut in some of these species. The hindgut of many carnivores includes a cecum, but neither the cecum nor colon are haustrated in most species.
Figure 5.11. Gastrointestinal tracts of a carnivorous monotreme, cetaceans, and Insectivora (echidna, dolphin, whale, elephant shrew , and mole). (From Stevens 1980; Stevens & Hume 1995.)
Figure 5.12. Gastrointestinal tracts of carnivorous Chiroptera, and Carnivora (insectivorous bat, vampire bat, mink, aardwolf, and dog). (From Stevens 1980; Stevens & Hume 1995.)
Figure 5.13. Gastrointestinal tracts of carnivorous Carnivora, marsupials, and primate (cat, seal, phascogale, quoll, and bushbaby). (From Stevens 1977; Stevens & Hume 1995.)
Mammals - Omnivores:
Figures 5.14, 5.15, and 5.16 show examples of the gastrointestinal tracts of omnivorous Insectivora, Carnivora, Marsupialia, Edentata, Rodentia, Artiodactyla, and Primates. With the exception of some omnivorous rodents, such as the rat, and some nectivorous and frugivorous bats, the stomach of omnivores and species that feed solely on plant concentrates is simple and noncompartmentalized. The intestine of omnivores varies in both its relative length and the ratio between midgut and hindgut. Bears have an extremely long intestine, with a short and indistinct hindgut, and the opossum intestine is almost equally divided between a midgut (small intestine) and hindgut (large intestine). The hindgut of many omnivores includes a cecum. The cecum and varying lengths of the colon are haustrated in some species, and the colon is haustrated throughout its length in the pig, human, and some monkeys. The human cecum becomes well-developed during gestation. However, by the time of birth it is represented by only a small distention in the proximal colon, which does not correspond to the apex of the fetal cecum, and a vermiform appendix.
Figure 5.14. Gastrointestinal tracts of omnivorous species of Insectivora, Carnivora, and Marsupialia (hedgehog, raccoon, bear, bandicoot , and opossum). (From Clemens 1980; Stevens & Hume 1995.)
Figure 5.15. Gastrointestinal tracts of omnivores species of edentate, rodent, and primates (armadillo, rat, squirrel monkey, vervet monkey, and macaque). (From Argenzio & Southworth 1974; Stevens 1980; Stevens & Hume 1995.)
Figure 5.16. Gastrointestinal tracts of omnivorous primates, and artiodactyla (baboon, chimpanzee, adult and fetal human, and pig). (From Argenzio & Southworth 1974; Wrong et al. 1981; Stevens & Hume 1995.)
Mammals - Herbivores:
The herbivorous panda has a simple stomach and extremely long midgut similar to that of the black bear shown in Figure 5.14. However, the gastrointestinal tract of most mammalian herbivores includes an expanded colon, cecum, or forestomach. An enlarged colon is the principal site for microbial fermentation in wombats, perissodactyls (equids, rhinos and tapirs), elephants, sirenians (manatees and dugongs), and herbivorous apes (Fig. 5.17). Haustrations extend over the cecum and the entire length of the colon of most of these species, and the proximal colon is additionally divided into permanent compartments in the perissodactyls and elephants.
Figure 5.17. Gastrointestinal tracts of herbivorous marsupial, primate, perissodactyls, and proboscidean colon fermenters (wombat, orangutan, pony, zebra, rhinoceros , and elephant). (From Stevens 1977; Clemens & Maloiy 1982; and Stevens & Hume 1995.)
The predominant feature in the gastrointestinal tract of most small mammalian herbivores such as the lagomorphs, herbivorous rodents, and arboreal marsupial herbivores is a large cecum, which serves as the principal site for microbial fermentation (Fig. 5.18). Although most primates have a well developed cecum, only a few are herbivores (Caton 1997).
Figure 5.18. Gastrointestinal tracts of herbivorous rodent, lagomorph and marsupial cecum fermenters (guinea pig, vole, rabbit, koala, and glider). (From Stevens 1977; Clemens 1977; and Stevens & Hume 1995.)
A large haustrated or compartmentalized stomach is the principal site for microbial fermentation in the sloths, macropod marsupials, colobid monkeys, and herbivorous artiodactyls (Fig. 5.19). Most of the forestomach of rat kangaroos consists of a cranial sac that serves for the immediate storage of ingesta, but the forestomach of kangaroos and wallabies consists principally of an enlarged, haustrated tubiform segment. The forestomachs of colobus and langur monkeys are also drawn into a series of sacculations or haustra, but the forestomachs of sloths and herbivorous artiodactyls are divided into a series of permanent compartments (see Fig. 5.9).
Figure 5.19. Gastrointestinal tracts of herbivorous edentate, marsupial, primate, and artiodactyl foregut fermenters (sloth, kangaroo, colobus monkey, sheep, and llama). (From Stevens 1977; 1980; 1983; Stevens & Hume 1995.)
Figure 5.9. Examples of mammals with an expanded forestomach; hippopotamus, llama, kangaroo, ox, and sloth. E designates esophageal entrance, P designates pylorus, 1 designates omasum, and 2 designates abomasum. (Modified from Stevens & Hume 1995.)
Hofmann (1973) classified the Ruminantia into bulk and roughage eaters that are relatively nonselective in their browsing or grazing, concentrate selectors that browsed on the more nutritious, succulent portion of plants, and an intermediate group whose feeding habits varied with the availability of food. Bulk and roughage eaters, such as cattle and sheep, tended to have the most well developed forestomach and smallest cecum. Concentrate feeders such as the dik-dik had the smallest rumen and omasum, and largest cecum. The Tylopoda, hippos, peccaries, and the Malayan pig deer also have a compartmentalized stomach, but lack the omasum.
Some herbivores show combinations of an expanded forestomach, cecum and hindgut (Fig. 5.20). Some rodents such as the guinea pigs and voles have both an expanded stomach and cecum. The hyrax have an expanded forestomach, cecum, and additional set of colonic appendages. Sirenians have an expanded stomach, pair of duodenal appendages, and an extremely long colon. However the cecum is the principal site of microbial fermentation these rodents, the cecum and colonic appendages are the principal sites in the hyracoids, and the colon is the principal site in the sirenians. The cecum can also serve as an important secondary site in some ruminants. It is an important site for fermentation in ruminant concentrate feeders such as the dik-dik, and arctic muskoxen have both a large forestomach and cecum.
Figure 5.20. Combination foregut and hindgut fermenters (hamster, hyrax, dik-dik, dugong). (Hyrax from Clemens 1977. Others from Stevens & Hume 1995.)
< Top of Page Next section: 6. Motor activity of the digestive tract