INTRODUCTION 23 



but the application to bacteriology was made by Koch. Since 

 their introduction successive advances in staining technic have in 

 every instance been followed by the discovery of new organisms 

 related to disease. The microscope, liquefiable media, and 

 anilin dyes constitute the trio of most important factors in the 

 development of the science of bacteriology. 



Development of Theories of Immunity. Knowledge that one 

 attack of certain diseases generally prevented a recurrence and 

 that diseases could not be indiscriminately transferred to all 

 species of animals has existed ever since the foundation of medi- 

 cine. Many theories have been advanced to account for this 

 phenomenon. A few of the names of investigators who have put 

 the facts into logical form for presentation and study should 

 be considered. Metchnikoff conceived that the white blood-cor- 

 puscles and some other body cells acted as scavengers and des- 

 troyed microorganisms in the blood. This theory in modified 

 form furnishes to-day the logical basis for many of the operations 

 of the practitioner. Von Behring (1890) published results of stud- 

 ies on the diphtheria bacillus in which he recounted the discovery 

 of the specific toxin of this organism and a specific antitoxin. 

 He laid the foundation for the present-day " humoral " theory 

 of immunity, which supplements so well the phagocytic theories 

 of Metchnikoff. Ehrlich has correlated and coordinated the vari- 

 ous facts of the humoral theory and has made substantial additions 

 to it. He has created a terminology which has been quite generally 

 accepted and has proved most useful in discussion of the sub- 

 ject. So extensive have been researches in the field of immunity 

 during the last two decades (since 1890) that it has assumed 

 almost the rank of a coordinate science. 



Development of Sanitary Science and Preventive Medicine. 

 In 1858 Murchison formulated the so-called pythogenic theory of 

 disease i.e., that disease is caused by the emanations arising from 

 decaying or putrefying organic matter, and by the consumption 

 of such materials in water and food. This theory was quite com- 

 monly accepted, and its practical applications form the basis for 

 our modern sanitary science. The disposal of sewage and refuse, 

 the purification of drinking-water, and adequate systems of plumb- 

 ing were advocated and adopted before the germ theory of disease 



