Variations in Predisposition. 17 



ogenic influence equally operative upon the whole group fails 

 to pcoduce its effect upon all, certain individuals resisting it suc- 

 cessfully ; or it is often noticed that here and there an animal 

 sickens under conditions quite favorable for most of its species. 

 The difference between different tissues in predisposition and im- 

 munity may be seen in comparing the skin and mucous mem- 

 branes. The surface of the skin, with its hard epithelial cover- 

 ing, is much less sensitive to irritative substances than are the 

 delicate mucous membranes ; the mucous membrane of the stom- 

 ach in the living animal is resistant to the action of acids, but 

 that of the lower end of the intestinal tract may be injured by 

 the acid of the gastric juice. 



A number of poisons (snake venom, tetanus toxine) are en- 

 tirely harmless when taken into the alimentary canal ; although, if 

 introduced by way of lesions of the skin into the tissues, they 

 are extremely pathogenic. 



Especial interest attaches to the differences in predisposition 

 and immunity toward the viruses of infectious diseases. In all 

 infectious diseases we have to deal with poisonous materials 

 caused by micro-organisms. These micro-organisms (micro- 

 phytes, vegetable microscopic organisms ; fnicro.coa, animal micro- 

 scopic organisms) gain entrance to the body by alimentation, or 

 by the respiratory path, or through ivounds, or may even actively 

 penetrate the tissues should they be in close relation with the 

 skin or mucous surfaces. Within the body structure they mul- 

 tiply for a time and work harm to the tissues chiefly by the 

 specific poisons existing in their protoplasm (that is, in the bodies 

 of the microbes) or by poisons elaborated by them, diffused in 

 the tissues and taken up by the blood. In the contest waged 

 with these microscopic foreign invaders a twofold task is set for 

 the animal body ; it must in the first place destroy the microbes 

 and at the same time must render their toxines inert. One ani- 

 mal may accomplish this with ease ; another with difliiculty. Some 

 species of animals are by nature uninfluenced by the toxine of an 

 infection which is sure to produce disease in another species ; 

 thus cattle are immune to glanders, the horse to pulmonary 

 tuberculosis, and chickens bear large doses of tetanus toxine with- 

 out injury to health. 



Such absence of reaction to infections and their toxines in an 

 animal species is spoken of as natural immunity. It is to be 

 explained from one standpoint by the idea that the cells and tis- 



