For comparison, see digestive system dissections of bats, a ferret, a fox, and a rabbit.

Dissection instructions & description

At this point your animal should be mostly skinned and cleared of fat. Insert a probe along the midline of the abdomen deep to the body wall musculature. In the next step, AVOID CUTTING THE ABDOMINAL OR PELVIC VISCERA. Pull the muscles taut, and cut along the probe with a scalpel or scissors. Reflect the body wall musculature laterally.

You will see a large liver. Caudally to the muscular diaphragm, and a coiled small intestine caudal to the liver. The diaphragm separates the thoracic compartment from the abdominal. The liver has three major lobes: central, left lateral, and right lateral. In ventral view you should be able to see the central lobe, divided into right central and left central lobules, and a small piece of the right lateral lobe (See Figure 1). [Web editor's note: Figures 1 and 2 could not be included on this page.] Look for the gall bladder in a large compartment of the right central lobule between the left central lobule and right lobe. In Figure 2, look at the magnified view of the gall bladder and bile duct. Reflect the liver and notice the large stomach, which is deep to and slightly to the right of the liver (See Figure 3). On the right side the pylorus attaches the stomach to the small intestine. Caudal to the stomach on the left look for the spleen. Between the spleen and the kidney observe the pancreas, which is a long strand of glandular tissue. From this point, the pancreas runs caudal to the stomach towards the duodenum, where it is pierced by the superior mesenteric artery. Cut the esophagus just superior to the stomach, and the large intestine just superior to the rectum. Also cut the superior and inferior mesenteric arteries and veins supplying the gut so that you do not damage the inferior vena cava or abdominal aorta. In Figure 4, the gut has been removed to provide a better view of the stomach, spleen, pancreas, small intestine, large intestine, and caecum. Note that all the coils seen in the superficial view of the gut are formed by the small intestine. The large intestine begins at the ileocolic valve, and runs for a few millimeters more to empty into the rectum (not pictured). The pouch-like structure located at the junction of the large and small intestine is the caecum. The very large small intestine relative to the large intestine and caecum indicates a highly nutritious diet (see below). The "small" in small intestine refers to its relative diameter, not length.

Abdominal viscera

The digestive tract of all mammals can be divided into three broad categories: foregut, midgut, and hindgut [1]. The foregut, composed of the esophagus and stomach, is responsible for storing, physically breaking down, and chemically digesting food. The midgut, or small intestine, is often highly coiled in mammals, and is responsible for continuing chemical breakdown and absorbing most nutrients. The hindgut, or large intestine, is usually larger in diameter and shorter in length compared to the midgut, and is responsible for taking up water and electrolytes and fecal formation. Finally, at the junction of the small and large intestine many mammals have a pouch, the caecum, responsible for fermentation. These general sections of the mammalian gut are usually specialized to suit the dietary requirements of particular species. Carnivores, who eat mainly a highly nutritious and easily digestible diet, have a simple digestive tract. The stomach and esophagus show few specializations other than a high degree of plasticity. Food might be encountered relatively infrequently and then consumed in large quantities or eaten rapidly placing a premium on small guts that can expand greatly. Carnivores frequently have a short, 2-6x body length, and undifferentiated small intestine accompanied by an attenuated large intestine and caecum. Animals with diets that are highly nutritious, such as blood, nectar, or insects, or animals that consume food almost constantly, filter feeders, also have very simple stomachs. Omnivores eat a wider variety of foods then the former group, but often eat a high percentage of high quality food items. Such animals tend to have a digestive tract that is slightly elongated and complex relative to faunivores. Herbivores on the other hand tend to eat extremely poor quality foods, that require large quantities of food and long digestion times. Another problem with herbivory is that mammals cannot break down cellulose on their own and require symbiosis with bacteria or protozoa for fermentation. Fermentation can occur in either the foregut or hingut (and very rarely in the midgut). Foregut fermenters usually have large and complex stomachs. Processing foods this way allows for a larger proportion of nutrients to be absorbed by the midgut, but takes an extremely long time. Most large herbivorous animals employ this method. Hindgut fermentation, on the other hand, requires a huge caecum, large intestine, or both. It is more rapid, but provides fewer nutrients because the food has already passed through the midgut. Hindgut fermenters eat larger quantities of food in a shorter period of time compared to their foregut counterparts. One advantage to this method is that harmful particles, such as tannins and silica, can be processed more quickly. Many hindgut fermenters, such as rabbits, eat their own feces in effect passing food through the gut twice in order to absorb a greater proportion of the potential nutrient stores.

I measured the length and approximated the diamter of the small intestine, large intestine, and caecum for several species from a wide array of taxonomic and ecological affinities. Since body mass or length was not known the measurements were converted to relative units based on the largest length or diameter for each species. In some cases, I simply took detailed notes where measurements were impossible or not necessary. Other than measurement error, this sample may be biased because of the extensive coiling of the small intestine, dessication, and loss of some of the large intestine during removal.

The dog and cat fit the carnivore/scavenger model very well. In the wild, cats and dogs are mostly carnivorous. Both species have an extremely reduced large intestine and caecum relative to the small intestine. It is possible that these domesticated animals could have eaten diets high in quality compared to their wild counterparts and may have even simpler guts.

The rabbit and lemur have the most extensive large intestine and caecum in this sample. Rabbits and lemurs are both known to exhibit some degree of hindgut fermentation. Both species also practice coprophagy for added nutrient absorption.

The opossum, Didelphis, and tree shrew, Tupaia belangeri, have values intermediate between the rabbit/lemur and dog/cat group. Opossums are omnivorous and ingest a large proportion of highly nutritious foods such as insects and fruit. In the wild, tree shrews have a diet consisting mostly of insects and fruit, and a small amount of fibrous material.

The sloth and goat fit the foregut fermentation model very well. Sloths eat mostly leaves and goats mostly grass. The goat has a large four-chambered stomach. Its small intestine is extremely long, thin, and highly coiled. The large intestine is not nearly as long as the small. At the caecal junction it is about 4 times the diameter of the small intestine but tapers after about a foot to 2 times the diameter within 20 cm. The caecum is extremely short in length relative to the small intestine, and about 4 times its diamter. The sloth has a highly coiled small intestine with little or no large intestine or caecum. Its stomach was much larger than those of the other species examined relative to the length of its intestine.

Bibliography

1 Hildebrand, M. and G.E. Goslow. 2001. Analysis of Vertebrate Structure. 5th ed. John Wiley and Sons, Inc. New York. pp. 201-217.

Additional anatomical references

Davis, D.D. 1938. Notes on the anatomy of the tree shrew Dendrogale. Field Mus. Publ. Chicago Zool. 20:383-405.

Le Gros Clark, W.E.1926. On the anatomy of the pen-tailed tree shrew (Ptilocercus lowii). Proceedings of the Zoological Society of London 1926:559-567.

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