8 BIOLOGICAL LECTURES. 



the toxic constituents of snake venom and the blood of certain 

 animals. The poisonous effects of snake venom are familiar 

 to you ; but it is less generally known that the blood of one 

 species of animal, freed from its corpuscular elements, may 

 exhibit poisonous properties when introduced in sufficient 

 amounts into an animal of a different species. Thus the blood 

 serum of human beings, and of the dog, is highly poisonous to 

 the rabbit, and that of the lamb to human beings. 



In the course of my studies in this direction, I have been 

 able to show that the injurious effects which the blood of one 

 animal produces in another is due to the toxic substances con- 

 tained in the foreign blood, and not alone to the destruction of 

 the blood-corpuscles of the host which commonly ensues. This 

 latter phenomenon is spoken of as the globulicidal in contra- 

 distinction to the toxicidal effects of the foreign serum, and 

 when the amount of foreign serum introduced into a susceptible 

 animal is very large, the destruction of corpuscular elements 

 may be great enough to cause immediate death. The toxicidal 

 effect is produced much more slowly. The toxic constituents 

 of the blood serum obtained either from the dog or from human 

 beings bring about in the tissues of the rabbit changes similar 

 to those caused by the vegetable toxalbumins which have been 

 considered. 



Having now seen that these agents of intoxication, whether 

 derived from animal forms of low or high position, or from 

 vegetable life high or low in point of organization, possess 

 certain features in common and produce similar pathological 

 effects when tested upon susceptible animals, it will be of 

 interest to extend a little further our inquiry into their 

 properties. 



Among the most interesting of these from a theoretical, and 

 of the greatest significance from a practical standpoint, are the 

 effects of small and repeated doses of these substances. The 

 facts elicited by the study of the effects produced by these 

 bodies when introduced into certain animals in this way have 

 an important bearing upon the questions of insusceptibility to 

 disease in general, and are included in the subject matter of 

 immunity. 



