All vertebrate digestives systems have the same basic plan. Food enters through the mouth and is usually broken into smaller pieces by teeth. Many vertebrates have a tongue that helps manipulate the food in the mouth. Most land vertebrates have salivary glands that lubricate the food and start the process of digestion.

The esophagus is a flexible tube that connects the pharynx and the stomach, a large pouch where food is stored and some digestion takes place.

Complete digestion and absorption take place in the small intestine, a long, slender, coiled tube leading from the stomach. Two large digestive glands, the liver and pancreas, as well as the smaller gallbladder, are connected by ducts to the anterior part of the small intestine.

Water and salt absorption, and the partial digestion of cellulose by bacteria, take place in the large intestine. Because animals don’t produce an enzyme that breaks down cellulose, these bacteria are very important in herbivores.

The large intestine ends in the anus.

The teeth of mammals are adapted to their diet: herbivores, such as deer, have cutting teeth, called incisors, that are modified for biting and cutting the leaves and stems of plants.

Herbivores also have ridged teeth, called molars, for crushing food and breaking down tough plant tissues.

A dog is a carnivore. Like a deer, it has incisors, but the dog’s incisors are associated with pointed teeth, called canines. Canines are used for ripping and shredding flesh, as well as for capturing and puncturing prey. In carnivores, modified molars are adapted to cut and chew the tough fibers of meat.

Humans are omnivores—we eat both plants and animals for food. Human teeth have some of the features of herbivores and carnivores. Like herbivores, we have closely meshing upper and lower molars for grinding. Like carnivores, we have canine teeth for tearing into a steak or a piece of crusty French bread.

The length of the mammalian digestive tract is also adapted to the animal's diet. A carnivore, like a dog, may have to go several days without food, so it gorges itself when food is available. The large stomach of such a carnivore is adapted for holding all the food it takes in. A dog also has a short large intestine—about 16 inches in a medium-sized dog.

The dog’s meat diet is easily digested by enzymes, and the processed food is passed rapidly through the digestive system. Notice the dog’s tiny cecum. This appendage is not very important in carnivores or omnivores.

A horse, an herbivore, has a small stomach compared to a dog. It eats continually and doesn’t need to store food. The much longer digestive tract of the horse helps it digest plant foods, such as hay and grass. You can see the horse’s large C-shaped cecum. The cecum of herbivores functions as a fermentation chamber where bacteria can break down plant food into a more digestible form.

Horses eat a plant diet that's relatively rich in nutrients. Other herbivores, such as sheep, cattle, and deer, eat a much poorer diet. These animals, called ruminants, have a modified digestive system for extracting enough nutrients. Most ruminants have a stomach with four chambers.

Chewed and swallowed tough plant material passes down to a stomach chamber called the rumen, where it is fermented by bacteria. After fermentation, the animal regurgitates the food and chews it some more. The combination of the mechanical breakdown of plant tissues by the teeth and the chemical breakdown by bacterial enzymes enables the animal to obtain the maximum amount of nutrients from its food. After rumination, the food travels to another stomach chamber called the omasum and eventually reaches the final chamber, the abomasum. The abomasum is the equivalent of our own stomach—the place where enzyme and acid digestion takes place.

Now that we’ve seen the components the digestive system of several vertebrates, let’s take a journey through the human digestive system to see how we digest our food.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education