18 VETERINARY BACTERIOLOGY 



insects, and other higher forms of animal life are likewise fre- 

 quently parasitic, but are excluded from consideration here. 



Medical bacteriology may be defined as that branch of bacteri- 

 ology which treats of those microorganisms (including the true bac- 

 teria, molds, and protozoa) that produce disease in the animal 

 body or are related directly or indirectly to the maintenance of 

 health. 



The history of veterinary bacteriology is closely linked with that 

 of general medical bacteriology, for many of the diseases of man 

 are transmissible to animals and vice versa. It should be remem- 

 bered that both are merely subdivisions of a great science, concern- 

 ing which it is important that the student should gain some- 

 thing of a perspective view, particularly with reference to its 

 history and development. This development has been so rapid, 



Fig. 1. Leeuwenhoek's drawings of bacteria: A, B, Bacilli, probably; C-D, 

 path of movement; E, cocci; F, Leptothrix, probably; G, spirillum. 



and so many of the important discoveries have been made within 

 recent years, that it is frequently difficult to determine their 

 relative importance. However, certain facts and personalities 

 stand out so conspicuously that they are deserving of brief con- 

 sideration. 



The Microscope and its Influence. The existence of living 

 plants or animals smaller than can be seen by the unaided eye 

 was conjectured by several of the Greek philosophers and phy- 

 sicians who used such theories in their speculations on the origin 

 and cause of fermentation and disease. Until the discovery 

 of the microscope such speculations were without any basis in 

 fact. 



Lccmvcnhook (1632-1723), in the course of his examinations 

 of a great variety of natural objects by means of the somewhat 



