SECTION IX 



DIGESTIVE SYSTEM 



Revised for the Fifth Edition 

 By C. M. JACKSON, M.S., M.D., 



PROFESSOR OF ANATOMY IN THE UNIVERSITY OF MINNESOTA 



IN order to furnish the living protoplasm with the materials necessary for 

 energy, growth and repair, a constant supply of food must be provided. 

 Most foods must be rendered soluble, and must undergo certain preliminary 

 chemical changes, in order to render them suitable for absorption and assimilation 

 by the cells of the body. For this preparation of the food-supply, the digestive 

 system [apparatus digestorius] is provided, which includes the alimentary canal 

 and certain accessory glands (salivary glands, liver and pancreas). The alimen- 

 tary canal is divided into a number of successive segments, varying in size and 

 structure according to their function. These segments (fig. 846) include the 

 mouth, pharynx, oesophagus, stomach, small and large intestines. 



Typical structure. — The most important layer of the tubular alimentary canal is the inner 

 mucous membrane [tunica mucosa]. From its epithelial hning, the various digestive glands are 

 derived, and through it the process of absorption takes place. The epithelium is supported 

 by a fibrous tunic [lamina propria mucoss] beneath which is a thin layer of smooth muscle 

 [lamina muscularis mucosse). The layer next in importance is the muscular coat [tunica muscu- 

 laris] which propels the contents along the canal. It is typically composed of two laj'ers of 

 smooth (involuntary) muscle, the inner circular and the outer longitudinal in arrangement. 

 Between the mucosa and the muscularis is a loose, fibrous submucous layer [tela submucosal, 

 which allows the folds in the mucosa to spread out when the canal is distended. Finally, there 

 is an outer fibrous coat [tunica fibrosa], which in the abdominal cavity becomes the smooth 

 serous coat [tunica serosa], or visceral layer of the peritoneum, which eliminates friction during 

 movements. The variations in the structure of the aUmentary canal in different regions are 

 due chiefly to differences in the mucosa. 



Glands. — Since the glands form an important part of the digestive system, the classifica- 

 tion of glands in general will be discussed briefly. A gland may be somewhat loosely defined 

 as an organ which elaborates a definite substance which is either a waste product to be eliminated 

 (excreted), or a secretion to be further utilized by the organism. Glands may be divided into 

 (a) ductless glands {e. g., spleen, thyreoid gland), which pour their secretions directly into the 

 blood or lymph; and (b) glands with ducts, which open upon an epithelial surface. Some organs, 

 however, belong in both classes (e. g., liver, pancreas). 



The glands with ducts (the so-called 'true' glands) are always derived from an epithelial 

 surface and may be further subdivided upon the basis of either (1) form or (2) ceU-structure. 

 According to form, glands are classified as either iu6uZar or saccuZar (alveolar, acinous). Each 

 of these may be either simple or com/pound (branched). The compound saccular form is often 

 called racemose. Moreover, intermediate forms (tubulo-racemose) occur. 



According to cell-structure and character of secretion, glands are divided into mucous 

 and serous types. In the mucous tj^pe, the cells appear larger and lighter (fig. 867) when swollen 

 with mucus which is secreted for purposes of lubrication. The goblet-cells of the intestine 

 represent imiceUular glands of this tj-pe. In the serous (or albuminous) type of glands, the 

 cells usually appear somewhat smaller and more deeply stained, with numerous zymogen 

 granules (fig. 867). The secretion is a watery, albuminous fiuid, which contains the digestive 

 enzymes. There occurs also a mixed type, with separate mucous and serous saccules, or both 

 tjTDes of cells may occur in the same saccule (the serous cells as 'demilunes' or 'crescents' 

 (fig. 867). In aU cases, the epithelial gland cells are supported by a fibrous connective-tissue 

 stroma, which provides a rich vascular and nerve-supply. 



Morphology. — The alimentary canal in comparative anatomy is divided into the head-gut 

 (mouth and pharynx) , fore-gut (oesophagus and stomach), mid-gut (small intestine), and hind- 

 gut (large intestine). EmbryologicaUy, the mid-gut corresponds roughly to the portion of the 

 archenteron attached to the yolk-sac, the portions of the archenteron anterior and posterior to 

 the yolk-sac being designated as fore-gut and hind-gut respectively. (See Section I, Morpho- 

 genesis.) The hning epithelium of the alimentary tract is endodermal, excepting the anal canal 

 and the mouth cavity, which are fined by invaginations of the ectoderm. 



In the region of the mouth and pharynx, the digestive and respiratory systems are closely 

 related in position, structure, function and origin. Morphologically, the head-gut represents 

 a primitive ahmentary-respiratory apparatus. 



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