DIGESTION 361 



it, although not necessarily to other secretions. It is easy to 

 multiply illustrations of this principle. 



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 are often disastrous. Leucocytes 

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

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

 section is dipped into it. The defensive, or in some cases, offen- 

 sive 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 lately been taken 

 by Weinland. Starting with the idea that if special protective 

 mechanisms against the digestive juices were anywhere to be 

 found it would be in the intestinal parasites whose whole exist- 

 ence 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 impregnated with them, 

 and it is then, just like the ' living tissue,' rendered for a longer 

 or shorter time unassailable by the proteolytic ferments. While 

 the supposed 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, these facts are 

 full of suggestion for future work. As already mentioned, it is 

 known that an antitrypsin exists in the blood, with the same 

 properties as the antitrypsin in -the intestinal worms (Hamill). 

 This explains the resistance of blood-serum to the digestive 

 action of trypsin. In addition to this body, which hinders the 

 action of fully-formed trypsin, and has . o effect upon entero- 

 kinase, the blood of some animals contains an antikinase i.e., a 

 substance which hinders the action not of trypsin, but of 

 enterokinase, preventing it from activating the trypsinogen into 

 trypsin. 



