298 - Multicellular Animals, Especially Man 



are named from within outward: (1) the 

 mucosa, (2) the submucosa, (3) the muscu- 

 laris, and (4) the serosa. In the frog's intes- 

 tine, for example, one finds the same four 

 layers as in the human digestive tract, al- 

 though the intestinal wall of the frog (Fig. 

 16-12) is relatively simple, compared to that 

 of man. The mucosa, or innermost layer of 

 the enteron wall, comes into direct contact 

 with the food in the digestive tract, and in 

 the frog the mucosa is represented mainly by 

 a single layer of glandular epithelium, which 

 is derived entirely from the endoderm (Fig. 

 16-12). 



LUMEN OF 

 INTESTINE 



EPITHELIAL CELL 



GOBLET CELL 



-MUCOSA 



BLOOD 

 VESSEL 



Fig. 16-12. 

 tine. 



SUBMUCOSA 



CIRCULAR 

 MUSCLE 



LONGITUDINAL"""^ ^SEROSA 

 MUSCLE (PERITONEUM) 



Simplified cross section of a frog's intes- 



The other three layers of the enteron wall 

 are all derived embryologically from meso- 

 derm. The submucosa consists mainly of 

 fibrous connective tissues, although it is 

 abundantly supplied with blood capillaries 

 and lymph vessels, into which the digested 

 foods are finally absorbed. The muscularis 

 (Fig. 16-12) consists of two sheets of visceral 

 muscle, which effect the movements of the 

 bowel. The inner part of the muscularis is 

 called the transverse layer, because its fibers 

 tend to encircle the enteron; but the fibers 

 of the outer part run lengthwise of the in- 

 testine, ami these make up the longitudinal 

 layer of the muscularis. The serosa, essen- 

 tially, is a single tier of flattened epithelial 



cells, which provides a smooth external lin- 

 ing for the enteron. A serious fluid is secreted 

 by the serosa, and this fluid lubricates the 

 abdominal surfaces, reducing the frictional 

 irritation as the coils of the intestine rub 

 against each other and against the other 

 organs of the abdominal cavity. 



Glands of the Digestive Tract. Many of 

 the cells of the mucosa are goblet cells (Fig. 

 15-12), which, essentially, are unicellular mu- 

 cous glands. Each goblet cell produces a 

 small globule of mucous periodically and 

 extrudes this secretion upon the food mass. 

 Collectively the mucosa produces consider- 

 able quantities of mucus, which serves as a 

 lubricant, facilitating the passage of food 

 through the digestive tract. 



The other digestive glands represent multi- 

 cellular outfoldings of the endodermal epi- 

 thelium. The gastric glands (Fig. 16-13) and 

 the intestinal glands are simple glands, in 

 that they are microscopic and lie entirely in 

 the wall of the digestive tract, but the salivary 

 glands, liver, and pancreas are compound 

 glands, and these large separate organs are 

 connected to the digestive tract only by their 

 ducts (Fig. 16-11). 



The Oral Cavity. The main structures of 

 man's oral cavity are quite familiar. The pal- 

 ate, or "roof of the mouth," is a partition 

 that separates the oral cavity from the nasal 

 passages (Fig. 16-11). The anterior bony part 

 of this partition is the hard palate, and the 

 posterior nonbony part is the soft palate, 

 which terminates as a fleshy projection, the 

 uvula. The oral cavity is guarded in front 

 by the lips, and flanked at the sides and in 

 front by the upper and lower sets of teeth. 

 The tongue, which is a flexible muscular 

 organ, arises from the floor of the mouth, 

 and the mucosa lines the oral cavity through- 

 out, except in the region of the gums and 

 lips, where the lining represents a sort of 

 modified skin. The oral mucosa is continu- 

 ously moistened by the mucous secretions of 

 its own goblet cells and by saliva, which 

 drains into the mouth from the salivary 

 glands (Fig. 16-1 1). 



