The wall of the alimentary canal is composed of four layers of tissue. The mucosa is a mucous membrane which lines the lumen or hole in the canal. The submucosa consists of connective tissue which binds the mucosa to the third layer, the muscularis. In the mouth, pharynx, and upper esophagus the muscularis is partly skeletal muscle. In the rest of the gastrointestinal tract the muscularis is composed of smooth muscle tissue. The muscularis of the stomach is composed of three layers of muscle tissue, while in the lower esophagus and intestines the muscularis has only two layers. The muscular layers of the muscularis run at angles to one another. In the intestine for instance there is an inner, circular layer and a deeper, longitudinal layer.

The deepest layer is the serosa, also known as the visceral peritoneum. The peritoneum has been discussed above.

Taste buds are embedded in the surfaces of fungiform and circumvallate papilla on the tongue as well as in the soft palate and throat. The taste buds contain gustatory cells which have chemically sensitive gustatory hairs on their free ends. The hairs protrude into the gustatory pore where they come in contact with chemicals dissolved in saliva. These cells generate nerve impulses which are sent to the brain. Between the gustatory cells are supporting cells.

Figure 20.4 Taste Bud

Gustatory cell Gustatory hair Gustatory pore Nerve fibers Support cell

There are four types of taste buds. Each is sensitive to a different type of chemical. Taste buds for sweet are located at the apex of the tongue. Taste buds for sour are on the sides. The taste buds for salt overlap both sour and sweet. The taste buds for bitter are located on the circumvallate papillae at the back of the tongue.

A cross section through a salivary gland reveals two kinds of cells, the mucus-producing cells and the enzyme-producing cells. The mucus helps to lubricate the food for swallowing while the enzyme salivary amylase begins the chemical digestion of starches.

A photomicrograph of the wall of the stomach shows narrow pits in the mucosa. These pits are known as gastric pits and are lined with mucus-producing cells. Deeper within the mucosa are two other secretory cells. The parietal cells secrete hydrochloric acid. The zymogenic cells secrete an enzyme involved in the digestion of protein. The enzyme is pepsinogen. The muscularis of the stomach has three layers rather than the usual two layers.

The wall of the small intestine has villi. The surface of the villus is composed of a single layer of columnar epithelial cells. Small glands deep to the villi secrete digestive juices into the intestine. Each columnar cell has microscopic folds of its cell membrane called microvilli. The microvilli further increase the surface area of the intestine allowing for greater absorption of nutrients.

The liver is composed of small functional units called lobules. Each lobule contains numerous canals which channel blood between the cells of the lobule into a central vein.

Exercise 20.4

Examine microscopic slides of: taste bud, salivary gland, stomach, small intestine, and liver. Note the following structures:

taste bud - gustatory cell, gustatory hair; gustatory pore, and supporting cell salivary gland - two types of cells stomach - gastric pit, parietal cell, and zymogenic cell small intestine - microvilli, simple columnar epithelium, and villus liver - central vein

Central Vein Small Intestines

Name the structures:

_ The teeth are held in the socket by cementum and

_ The papillae at the back of the tongue are (2) papillae.

The bones which make up the hard palate are the two palatine and two (3) .

Name the salivary glands in order from posterior to anterior.

Which part of the peritoneum holds the liver to the diaphragm?

The folds inside the stomach are the (8) .

Name the three divisions of the small intestine in order.

The layer of the GI tract which is located between the muscularis and the mucosa is the (12) .

What is the function of the greater omentum?

The inferior end of the ascending colon is the (14).

Which duct empties the gall bladder?

Name the tissue:

Name the tissue:


Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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