THE RELATIONS OF PATHOLOGY 113 



at any time for the purpose of investigation and better insight 

 obtained into the evolutions of morbid processes. The disease may 

 be studied in all its phases. Hence comparative pathology rapidly 

 became the refuge of the investigator finding his way blocked by 

 the necessary restrictions governing the study of human diseases. 

 The great influence of the comparative method of study of infectious 

 diseases is well shown in the relatively advanced state of our know- 

 ledge in regard to those human diseases of this class that are readily 

 communicable to animals as compared with our ignorance in regard 

 to the cause of certain other human diseases which so far as we know 

 are not transferable to animals. 



As the secrets of the vast domain of parasitism were revealed, 

 and the teachings of specific etiology and pathogenesis became 

 appreciated, there sprang up in the place of the therapeutic hope- 

 lessness inspired by the study of pathological anatomy only, an in- 

 creasing interest of enormous consequences in preventive measures. 

 This was the natural outcome of the persistent efforts now made 

 to follow the chain of causation so far as it was possible to go; for 

 it early became established that the farther back of the immediate 

 causes of diseases we can come the more easily and economically are 

 they controlled and, reversely, the nearer we approach the period 

 in the evolution of disease characterized by open manifestations the 

 more difficult is disease to overcome. Hence the newer ideas of 

 cleanliness, of surgical asepsis, sanitary science, and preventive 

 medicine, all are the offspring of the study of microbiology and 

 etiology in a wide sense. Indeed, the great principle of prevention 

 may be applied with perfect success even when the actual cause of 

 the disease remains unknown. The discovery by Walter Reed, for 

 instance, that the cause of yellow fever is conveyed by a certain kind 

 of mosquito makes it possible to prevent this destructive disease 

 with absolute certainty by destroying the mosquito or preventing 

 its bite. 



Interaction of Parasite and Host Bio-chemistry and Immunity. 



But the fundamental problems of etiology are not wholly solved 

 by the discovery of the causative agent, however important this 

 step may be; for it remains to explain how normal function and 

 structure are upset by the entrance of this new factor. 



Now the study of bacteriology and comparative pathology has per- 

 mitted a deeper penetration into the nature and mechanism of cer- 

 tain infections. The discovery of bacterial and other toxins, complex, 

 soluble, and diffusible chemical substances, and of their wonderful 

 influence upon the metabolism of cells, opened new and rich fields 

 that under the hands of keen investigators have furnished precious 



