PRINCIPLES OF VETERINARY SURGERY 151 



lactic value, is conceded by the entire scientific world. The 

 controversy relates wholly to the manner of their develop- 

 ment and action, and these theories are far from speculative. 



The study of immunity has revolutionized therapeutics 

 to such an extent that more than one-half of the indicated 

 treatment in infectious diseases is with immune substances. 

 In prophylaxis, some idea can be had of the extensive usage 

 of antitoxin by the fact that during the last ten years over 

 50,000,000 doses of blacklegoids have been given. In many 

 diseases, the diagnosis is wholly dependent upon the mani- 

 festation of substances formed in the body during the process 

 of infection. 



As will be seen from the foregoing, the study of immun- 

 ity includes etiology; the preparation and administration of 

 serums and vaccines ; serums and toxins in the diagnosis of 

 disease ; the agglutinins, precipitins, lysins and opsonins ; 

 and the theories of the manner by which these bodies act, 

 namely, Metchnikofif's theory of phagocytosis and Ehr- 

 lich's side-chain theory. The older hypotheses have either 

 proven to be erroneous or else have been modified to har- 

 monize with those of Ehrlich and Metchnikofif, and hence 

 will not be considered here. And, as will be seen later, both 

 the theories of Ehrlich and Metchnikofif, with a few possi- 

 ble exceptions, are but different parts of the same hypothe- 

 sis. 



HISTORY. — Immunity is not a recent discovery, hav- 

 ing been known for many centuries. It has been known that 

 certain animals are not susceptible to some diseases, such 

 as a horse to symptomatic anthrax, cattle to glanders, etc. ; 

 and that an attack of a certain infectious disease brings 

 about such a change in the animal's tissue as to protect it 

 from further attacks of the same disease. It has also been 

 well known that certain diseases which attack man cannot 

 be very easily inoculated into the lower animals. Not only 



