THE alimentary tube is a long continuous tube with an inlet at the mouth and an outlet at the anus (Fig. i).
From a functional point of view, it is possible to divide it broadly into three segments. The first segment extends from the inlet of the tube to the middle of the duodenum and may be described as preparatory; here the foodstuffs are mechanically and chemically prepared for presentation to the next segment, but are not sufficiently altered to allow of any absorption. The second segment extends from the middle of the duodenum to the middle of the transverse colon; in it all the main digestive and absorptive processes are accomplished. The third segment, which extends from the middle of the transverse colon to the outlet of the tube, may be described as expulsive; it is almost entirely devoted to the rejection of such portions of the foodstuffs and other waste materials as cannot be utilized by the body.
For a more detailed study of the structure and functions of the alimentary tube, it is convenient to divide it into a series of compartments each presenting definite differences in structure that can be related to specific functions (Fig. 2). These compartments are as follows:—
- Compartment I.—Lips to the faucial isthmus.
- Compartment II.—Faucial isthmus to the cardiac orifice of the stomach.
- Compartment HL—Cardiac orifice of the stomach to the middle region of the duodenum.
- Compartment IV.—Middle of the duodenum to the middle region of the transverse colon.
- Compartment V.—Middle of the transverse colon to the anal canal.
- Compartment VI.—The anal canal.
Attention is drawn particularly tç the fact that in this description the anatomical division of the abdominal part of the alimentary tube into stomach, small intestine, and colon is replaced by one which closer study will show con- forms more intimately to embryological, anatomical, and physiological findings.
The fundamental pattern of the wall of the tube may be studied readily in a transverse section taken from the lower part of the small intestine. The outermost coat is the peri- toneal covering—tunica serosa. Under this is the muscular coat—tunica muscularis—arranged in two layers, namely, an outer of longitudinal and an inner of circular fibres. The innermost coat of the tube is the mucous membrane—tunica mucosa—and in it may be recognized: (i) A surface layer of epithelial cells and simple glands resting on a basement membrane; (2) A layer of retiform tissue containing scattered lymph-cells; (3) A layer of very thin unstriped muscular tissue—the muscularis mucosae. The tunica muscularis is connected to the tunica mucosa by a loose but strong layer of areolar tissue—the submucous layer—in which run the larger blood-vessels and nerves of the wall of the tube.
The fundamental pattern undergoes modifications in the separate parts of the alimentary tube, and these will be indi- cated as they are encountered in the course of the description.
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