muscularia J 



DIGESTIVE SYSTEM 725 



cells. Starting from the inner layer we find them in the following order 

 (Fig. 291): 



mucosa 

 submucosa 



circular 

 longitudinal 

 serosa 



It is in the mucosa or the inner layer that the glands which produce 

 the digestive juices are found. Here, too, are a few scattered involuntary 

 muscle fibers and lymphatic vessels for carrying away the nutriment after 

 it has been changed into a condition so it can be assimilated. 



The submucosa is a thin layer of connective tissue supporting the 

 mucosa. 



The musculosa varies a good deal, but essentially it is composed of 

 two layers of involuntary muscle cells both circular and longitudinal, the 

 former lying toward the lumen. 



The circular muscles by contracting, lengthen the intestines while 

 the contraction of the longitudinal muscles shorten and thicken it. These 

 two actions cause the peristaltic movement occurring during digestion, 

 pushing the food forward and also permitting the various folds and little 

 finger-like projections, called villi, to come in contact with all of the ma- 

 terial that has been ingested. 



In the higher forms, especially in the human being, there' are some 

 thirty feet or more of small intestine as contrasted with three or four feet 

 of large. The reason for this can be understood quite readily when it is 

 appreciated that a two-inch water pipe holds eight times as much as a 

 pipe one inch in diameter. The great mass of material that is ingested 

 is of no value whatever to the animal ingesting it unless such food can 

 be reduced to a more or less liquid state and be absorbed by the mucous 

 lining of the intestinal tract. Digestion, though beginning in the stom- 

 ach, really takes place in the small intestine. The smaller this intestine 

 is in diameter, therefore, and the more folds the mucosa has, the more 

 readily will the food, after it is sent through the digestive canal, be likely 

 to come in contact with the mucosa and be absorbed. 



It is important to understand this as it will throw much light upon 

 various physiological functions of digestion, for, it will be seen that the 

 little finger-like villi must actually do the work of absorbing. That is, 

 after ingested material is ready for absorption, it does not pass by any 

 rule of gravity or mechanics into any definite opening; but these little 

 projections must actually reach out and drink in the necessary material. 

 As these little villi must in turn be kept in good condition and capable of 

 performing their functions by their nerve and blood supplies, it follows, 

 that where the nerve and blood supply is either weakened or lost, the 

 animal may die of starvation regardless of how much food it may ingest. 

 Many glands are found in the mucosa. Some of the larger ones have 



