Structure of the Wall

The wall of the alimentary canal consists of four distinct layers that are developed to different degrees from region to region. Although the four-layered structure persists throughout the alimentary canal, certain regions are specialized for particular functions. Beginning with the innermost tissues, these layers, shown in figure 17.3, include the following:

1. Mucosa, or mucous membrane (mu'kus mem'bran). This layer is formed of surface epithelium, underlying connective tissue (lamina propria), and a small amount of smooth muscle (muscularis mucosae). In some regions, it develops folds and tiny projections, which extend into the lumen of the digestive tube and increase its absorptive surface area. It may also contain glands that are tubular invaginations into which the lining cells secrete mucus and digestive enzymes. The mucosa protects the tissues beneath it and carries on secretion and absorption.

2. Submucosa. The submucosa contains considerable loose connective tissue as well as glands, blood vessels, lymphatic vessels, and nerves. Its vessels nourish the surrounding tissues and carry away absorbed materials.

3. Muscular layer. This layer, which provides movements of the tube, consists of two coats of smooth muscle tissue. The fibers of the inner coat encircle the tube. When these circular fibers (they are actually closed spirals) contract, the diameter of the tube decreases. The fibers of the outer

Mouth

Tongue

Tooth

Parotid salivary gland

Pharynx

Submandibular salivary gland

Mouth

Tongue

Tooth

Sublingual salivary gland

Parotid salivary gland

Pharynx

Submandibular salivary gland

Esophagus

Liver

Esophagus

Liver

Small intestine

Anal canal

Blood Vessels Teeth

Stomach

Pancreas

Rectum

Figure 17.1

Major organs of the digestive system.

Gallbladder

Duodenum (of small intestine)

Large intestine

Small intestine

Stomach

Pancreas

Rectum

Anal canal

Figure 17.1

Major organs of the digestive system.

muscular coat run lengthwise. When these longitudinal fibers (open spirals) contract, the tube shortens.

4. Serosa, or serous layer (se'rus la'er). The serous layer, or outer covering of the tube, is composed of the visceral peritoneum, which is formed of epithelium on the outside and connective tissue beneath. The cells of the serosa protect underlying tissues and secrete serous fluid, which moistens and lubricates the tube's outer surface so that the organs within the abdominal cavity slide freely against one another.

Table 17.1 summarizes the characteristics of these layers. Movements of the Tube

The motor functions of the alimentary canal are of two basic types—mixing movements and propelling movements (fig. 17.4). Mixing occurs when smooth muscles in small segments of the tube contract rhythmically. For example, when the stomach is full, waves of muscular contractions move along its wall from one end to the other. These waves occur every twenty seconds or so, and they mix foods with the digestive juices that the mucosa secretes.

Propelling movements include a wavelike motion called peristalsis (per"i-stal'sis). When peristalsis occurs, a ring of contraction appears in the wall of the tube. At the same time, the muscular wall just ahead of the ring relaxes—a phenomenon called receptive relaxation. As the wave moves along, it pushes the tubular contents ahead of it. Peristalsis begins when food expands the tube. It causes the sounds that can be heard through a stethoscope applied to the abdominal wall.

A device the size of a medicine capsule can image the alimentary canal, revealing blockages and sites of bleeding. The patient swallows the capsule, which contains a camera, a light source, radio transmitter, and batteries. About 6 hours later, it transmits images from digestive headquarters — the small intestine — to a device worn on the physician's belt. The information goes to a computer, and still or video images are downloaded. The device, which is disposable, leaves the body in the feces within a day or two. The "gi camera" is based on a device called a Heidelberg capsule used to monitor stomach acid. Soon to come is a capsule with longer-lasting batteries and better light to image the large intestine.

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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