The Digestive System



The digestive system of the lamprey, posterior to the pharynx, is a simple tube. The first portion is called the esophagus, although it is not greatly different in its external appearance from the intestine which follows. Inside the intestine is found a modified fold of the intestinal lining called a typhlosole, or spiral valve. It forms a long curtain-like partition which is suspended into the lumen of the intestine through most of its length. This long typhlosole's attachment to the intestine wall is mid dorsal at the anterior end, then spirals slightly as it progresses toward the posterior end.


In sharks, other chondrichthians, polypterids, sarcopterygians, sturgeons, paddlefish, bowfins, and gars, the intestine retains the spiral partition. Its spiral is more pronounced in these fish than in the lamprey.

These other fish have a stomach between the esophagus and intestine, complete with cardiac and pyloric regions, an enlarged lumen, and ususally a "J" shaped stomach body. The rectal gland seen in chondrichthians helps to remove salt from the body tissues. Intestines, urinary ducts, and reproductive passages terminate at a common space known as a cloaca in most fishes and tetrapods. Exceptions include lampreys, holocephalans, coelacanths, and therian mammals.

Higher teleost fish loose the spiral valve modification of the intestine in favor of a lengthened system which coils about within the body cavity. Many teleosts have pyloric ceca, which are small diverticula of the intestine located just posterior to the pyloric sphincter.

In the amphibians, and all tetrapods, the intestine becomes divided into a small and a large intestine.

In Birds there is usually a crop. The crop is a sack-like outgrowth from the lower esophagus which stores the swallowed food before it is sent to the stomach. The center of the stomach is dominated by a tough muscular region known as a gizzard. The area of the stomach above the gizzard is known as the proventriculus. A pair of ceca are found attached to the intestines near the junction of the small and large intestines.

Mammals have three segments to their small intestine, although these are not visible from the outside. The duodenum is the first segment of the small intestine. It contains special mucous glands in its submucosal layer. The second segment is the jejunum. The third segment is the ileum, which is characterized by abundant lymphatic nodule tissues in its wall. Some mammals have a large cecum, some have a small appendix to the cecum, some will have a pair of ceca. In marsupials and placental mammals the large intestine does not terminate in a cloaca.


Introduction to Human Digestive System

Human Oral Cavity

Structure of a Tooth

Tooth Development

Human Salivary Glands

Divisions of the Pharynx

Human Esophagus

Human Stomach

Human Small Intestine

Human Large Intestine

Human Biliary System and Pancreas

Mouth Development

Formation of Pharyngeal Derivatives

Development of Liver, Gall Bladder, and Pancreas

Development of Intestines

Urorectal Fold and Separation of Cloaca

A Comparative Look at Vertebrate Teeth

A Comparative Look at the Vertebrate Tongue

A Comparative Look at Digestive Tubes