38 MEDICINE 



rupted at any stage and the conditions studied at this stage. We 

 know the infectious diseases of animals chiefly by their experi- 

 mental production. There has been but little study of these dis- 

 eases under natural conditions and much knowledge can be gained 

 by the mode of, and conditions predisposing to, infection. Ques- 

 tions of heredity have an important bearing on disease. The sus- 

 ceptibility of animals to disease varies. Common experience has 

 shown in man also that, under circumstances apparently the same, 

 certain individuals will acquire diseases, others remain exempt. 

 There is also foundation for the belief that susceptibility for cer- 

 tain infectious diseases is inherited and in other diseases inherited 

 susceptibility is beyond doubt. The most striking recent discovery 

 in medicine is that the blood-serum contains many complex sub- 

 stances. Some of them play an important role in the animal econ- 

 omy, for others we can as yet discern no purpose, and our know- 

 ledge of these substances is chiefly confined to their effects, but it 

 has recently been found possible to isolate one substance in pure 

 form with a known chemic composition. While these substances 

 may serve an important role in protecting the body against disease 

 they may act in the opposite way by providing a means by which 

 injurious substances are brought in contact with cells. Whether 

 chemic variation may not arise, be inherited, and play an important 

 part in disease susceptibility is an important question to be answered 

 by comparative medicine. For the purpose of such investigation 

 an animal clinic is necessary, which should be provided with thorough 

 facilities for the study of disease. The questions for solution should 

 come both from comparative medicine and from the clinic of human 

 disease. 



Comparative medicine is intimately associated with experimental 

 medicine. There can be no contention as to the relative advan- 

 tages of observation and experiment. The experiment is only obser- 

 vation under simple and known conditions and supplements obser- 

 vation under the more complicated natural conditions. In the 

 experiment it is possible to divide questions into their simpler com- 

 ponents and make each the subject of experiment. In experimental 

 medicine just as in the animal clinic, the questions for solution should 

 come from both comparative medicine and the human clinic. The 

 most brilliant results in experimental medicine have come from the 

 study of the infectious diseases. Knowledge of these diseases stands 

 in direct relation to the possibility of their experimental production. 

 It is true that we have not been able to produce in animals many 

 of the diseases which are found in man. Experimental medicine 

 is comparatively new and the number of animal species experi- 

 mented upon has not been large. It has recently been found possible 

 to produce syphilis in the chimpanzee and there is every reason to 



