MICROORGANISMS IN NATURE 23 



in the lower animals only, as, for instance, the microorganisms of 

 Texas fever of cattle, black-leg of cattle, diphtheria of calves, leg 

 and lip diseases of sheep, etc., many other microorganisms cause 

 identical or very similar diseases both in man and the lower animals. 

 To these latter belong the pathogenic microorganisms which produce 

 such common diseases as tuberculosis, actinomycosis, glanders, 

 tetanus or lock-jaw, inflammations, suppurations, blood-poisoning, etc. 

 Hence, studies concerned with disease-producing microorganisms in 

 their relation to human and to veterinary medicine overlap, and the 

 general underlying principles and the methods employed for the 

 elucidation of the subject are identical. It is, of course, obvious 

 that in a book primarily designed for veterinary students and practi- 

 tioners, pathogenic microorganisms will be taken up especially with 

 reference to diseases of the domestic animals. Reference to human 

 diseases wnll only be made in a brief manner, in so far as is necessary 

 to point out sufficiently certain common features and to emphasize 

 the possibilities and the dangers of the transmission of diseases of 

 domestic animals to human beings, and vice versa. This is a subject 

 in which the veterinarian is interested both from a personal standpoint 

 and on behalf of the community. In other words the modern scien- 

 tific veterinarian must be a hygienist, not merely for the benefit of 

 his patients, the domestic animals and their owners, but also for 

 the benefit of mankind at large. 



Non-pathogenic Microorganisms. A limited number of microorgan- 

 isms which are not disease-producers, and which, from a medical 

 standpoint, are entirely harmless, will also be considered briefly. Such 

 microorganisms are used in the laboratory training of the student to 

 familiarize him with morphologic features and technical methods 

 early in his studies, at a time when it would not be advisable to give 

 into his unpractised hands dangerous, live, disease-producing micro- 

 organisms, and also because some harmless widespread bacteria are 

 very similar to certain pathogenic bacteria. The student must learn 

 to distinguish such harmless bacteria, as, for instance, the common 

 hay bacillus or B. subtilis, from dangerous pathogenic bacteria like 

 the anthrax bacillus, because they are very similar in their morpho- 

 logic and cultural features. 



Microorganisms Causing Fermentation. Some microorganisms which 

 are important in producing fermentative changes, both desirable and 

 undesirable, in milk, cheese, and other organic materials, will also be 

 briefly considered. These are also matters in which the veterinarian 

 is likely to be consulted and in which he should be at least sufficiently 

 well versed to form an intelligent valid opinion. 



Microorganisms in Nature. Microorganisms are, of course, not all 

 disease producers; in fact, the great majority of them live in the 

 outside world not merely a harmless but a very useful existence. 

 Certain classes are the cause of necessary and useful fermentative 

 processes; while others bring about the decomposition of dead organic 



