PREDISPOSITION, IMMUNITY, AND DISEASE. 383 



own secretion, as well as to a certain degree against various sick- 

 ening external influences. 



Prominent naturalists are at present occupied in inquiring for 

 a reasonable way of interpreting the causes of sickness and the con- 

 ditions of immunity from it, or the resistance offered by a sound 

 organism. Sickness, as well as health, according to one of the 

 prevailing theories, depends upon chemical causes, viz., on the 

 presence and predominance of various complex substances gene- 

 rated in the juices and tissues of the body by unknown processes, 

 in which bacteria may sometimes play an important part. Ac- 

 cording to another theory, the living animal cells are engaged in 

 a continual struggle against intruding micro-organisms. Animal 

 cells are considered as individuals similar in character to the order 

 of Amc&bcR, which are unicellular organisms of the class of Pro- 

 tozoa. Metschnikoff found that certain cells of the animal body 

 are endowed with the faculty of swallowing and digesting in- 

 truding bacteria of every kind, harmless ones as well as patho- 

 genic ones, or such as produce disease. Not all elementary organs 

 of the body are equally qualified for this purpose, the function 

 being intrusted to certain cells of the tissues and blood, which 

 Metschnikoff calls phagocytes. Health as well as disease de- 

 pends upon which party is victorious in the struggle. Health is 

 insured as long as the cells are capable of overpowering the in- 

 truding bacteria ; an animal in such a condition is secure against 

 disease. Experiments performed by Metschnikoff have given evi- 

 dence that the bacilli of splenic fever are easily devoured and 

 digested by phagocytes. On the other hand, several observers of 

 late have maintained that the liquid part of blood, the plasma, 

 and even common albumen, possess the faculty of killing bacteria. 

 This, however, appears improbable, and a final decision of the 

 question has still to be expected in future. 



Susceptibility to diseases is as variable as sensitiveness to vege- 

 table and animal poisons. Judging from the current opinion that 

 putrefying animal matter is the principal bearer and transport- 

 er of infectious germs, we are forced to ascribe a high degree of 

 immunity to certain animals which, like swine, ducks, chickens, 

 and rats, are accustomed to select their food from places where 

 such matter is accumulated. Predisposition for splenic fever is 

 stronger among herbivora than among carnivora ; birds of prey 

 seem to be quite free from it. Experiments on sheep, performed 

 by Pasteur, the results of which were confirmed by application 

 on a large scale, gave evidence that immunity against splenic 

 fever may be acquired by systematic inoculation of the attenuated 

 virus very much as small-pox is prevented by vaccination. 



Various herbivora, chiefly horses, sheep, and goats, are exposed 

 to a disease called " glanders," which ends by death in most cases. 



