18 



CHAPTER III. 



iMMUNIir iND HbrBDITT. 



In this chapter I shall try to give a sketch of the respective theories iif 

 immunity and heredity, both of which have a large influence on disease. 

 Susceptibility is, of course, the opposite of immunity. 



Immunity. 



Immunity is the povper v^hich an individual or a species has to resist the 

 attack of a disease. It may be racial, individual, or acquired ; and may be 

 absolute or comparative. For instance, horses possess racial immunity 

 against the scarlet fever of man and the pleuro-pneumonia of cattle. Men 

 are immune from sitrangles ; and ruminants from glanders. Among every 

 species of animal, we find individuals which are more or less refractory to 

 the inroads of a disease to which their fellows show marked susceptibility. 

 The man, horse, or ox which has had one attack, respectively, of smallpox, 

 strangles, or pleuro-pneumonia, will have acquired more or less immunity 

 from a second attack. This acquired immunity is never absolute. 



As possessors of comparative immunity, I may cite Algerian sheep, which 

 are refractory to anthrax, when inoculated, in the usual way : but prove 

 susceptible to it when the dose of the virus is largely increased. 



Dogs rai'ely contract tetanus ; horses, on the other hand, are very sus- 

 ceptible to it. 



On page 125, I allude to the accidental immunity of the carnivora from 

 actinomycosis. 



The immunity of fowl against anthrax appears to be due to their high 

 internal temperature (107.5° F.) ; for Pasteur has proved that if a fowl be 

 inoculated with anthrax, and is then placed and kept in water at a tempera- 

 ture of 77° F., so as to considerably reduce its internal heat, it will die of 

 anthrax in about a day and a half. 



The nature of the lesions set up in the tissues by certain diseases, greatly 

 affects comparative resistance. Thus, in a man who has been inoculated 

 with anthrax, the virus is more or less arrested at the seat of inoculation in 

 the malignant pustule which forms at that spot ; but in the horse it spreads, 

 apparently unchecked, throughout his entire body. Hence, anthrax is far 

 more fatal, and runs a much more rapid course in horses than in men. 



In those diseases in which one attack has a well-marked power of con- 

 ferring subsequent immunity, protection, according to Metchnikoff, is 

 acquired by the previous training in devouring microbes which the 

 leucocytes have undergone. "If we inoculate the virus of anthrax, for 

 example, under the skin of an ordinary rabbit and of a rabbit which has 



