68 DIGESTION 



Self-digestion of the Alimentary Tract. Why the stomach 

 or ar\y other portion of the intestinal tract brought into con- 

 tact with digestive juices is not destroyed has given rise to 

 much discussion. Normally, self-digestion does not occur, 

 but if an animal is killed while in full digestion and the body 

 is kept warm, the stomach will be destroyed. This has been 

 found to take place in human bodies. If a portion of the stomach 

 is deprived of its blood supply, that portion will be attacked 

 and a perforation of the stomach may result. The immunity 

 of the stomach to the gastric juice has been explained in a 

 number of ways, but not satisfactorily. It has been said that 

 the epithelial lining of the stomach prevents the absorption 

 of the gastric juice, but this explanation raises the question 

 why the living epithelial cells are immune. 



The secretion of mucus forming a protective coating for 

 the stomach is an inadequate explanation, owing to the diffi- 

 culty of conceiving such a means of protection to be as perfect 

 as it is. Another theory which holds the alkaline blood to 

 neutralize the acid of the stomach as it is formed cannot be 

 applied in the case of the intestine, where the digestive juice 

 is alkaline. An explanation is at present impossible. All 

 that can be said is that the immunity of the intestinal tract 

 is due to the fact that it is alive. It has been shown by Neu- 

 meister that a living frog's leg is not digested by a strong 

 pancreatic digestive mixture of weak alkaline reaction, because 

 in this case the cells are not killed. Bernard introduced - the 

 hind leg of a living frog into a dog's stomach through a fistula. 

 It was digested. But in this case the cells of the frog's limb 

 were first destroyed by the acid. 



There is some reason for believing that free hydrochloric 

 acid cannot penetrate living cells and free hydrochloric acid 

 must be present before peptic digestion can begin. In some 

 gland cells the enzyme is present within the protoplasm in an 

 inert zymogen form, as in the case of the pancreas. When 

 trypsin is injected under the skin it causes the tissues to break 

 down and ulcerate. Active trypsin may remain for a long 

 time in an isolated loop of small intestine without producing 

 any serious consequences ; but if the intestinal wall is previously 

 damaged, not only is the tract itself affected, but soon the liver 

 as well. Few tissues can bear the constant contact of urine 



