384 DIGESTION 



in contact with it, although not necessarily to other secretions. 

 It is easy to multiply illustrations of this principle. 



The mucosa of the dog's urinary bladder is digested by the 

 natural activated pancreatic juice of the dog, and still more readily 

 by the natural gastric juice. Yet few tissues but the lining of the 

 urinary tract or of the large intestine could bear the constant contact 

 of urine or faeces. When urine is extravasated under the skin, or 

 the contents of the alimentary canal burst into the peritoneal 

 cavity, they come into contact with tissues which, although alive, 

 are much less fitted to resist them than the surfaces by which they 

 are normally enclosed; and the consequences, which are not wholly 

 due to infection, are often disastrous. Leucocytes thrive in 

 the blood, but perish in urine. Blood does not harm the endo- 

 thelial cells of the vessels, but kills a muscle whose cross-section 

 is dipped into it. The defensive or, in some cases, offensive 

 liquids secreted by many animals are harmless to the tissues 

 which produce and enclose them. A caterpillar investigated 

 by Poulton secretes a liquid so rich in formic acid that the mere 

 contact of it would kill most cells. The so-called saliva of Octopus 

 macropus contains a substance fatal to the crabs and other animals 

 on which it preys. The blood of the viper contains an active 

 principle similar to that secreted by its poison-glands, but its tissues 

 are not affected by this substance, so deadly to other animals. 



A step in the solution of our problem has been taken by Wein- 

 land. Starting with the idea that if special protective mechan- 

 isms against the digestive juices were anywhere to be found, it would 

 be in the intestinal parasites whose whole existence is passed among 

 them, he has made the important discovery that in these parasitic 

 worms specific antiferments exist i.e., substances which inhibit the 

 action either of pepsin or of trypsin or of both. These substances can 

 be precipitated from the expressed juice of the worms by alcohol, 

 without completely losing their activity. Fibrin can be impreg- 

 nated with them, and it is then, just like the ' living tissue,' rendered 

 for a longer or shorter time unassailable by the proteolytic ferments. 

 These facts are full of suggestion for future work, although the sup- 

 posed proof that similar antiferments are contained in the cells of 

 the mucous membrane of the stomach and intestines of the higher 

 animals appears to have broken down. Substances can indeed be 

 obtained by Weinland's method from the gastric and intestinal 

 mucosa which, when added to a digestive mixture, strongly inhibit 

 the digestion of proteins. But there is no clear proof that these sub- 

 stances are specific antiferments. They are probably merely some 

 of the split products of protein (Langenskjold). There is, however, 

 some evidence of the existence of an antipepsin in many tissues 

 including the mucous membrane of the stomach. As already men- 

 tioned, it is known that an antitrypsin exists in the blood, with the 



