110 CLINICAL BACTERIOLOGY. 



DETERMINATION OF THE PATHOGENICITY (OR THE SPE- 

 CIFICITY) OF BACTERIA BY ANIMAL EXPERIMENTATION. 



The possession of pure cultures renders it possible to 

 undertake experiments upon animals, and thus to deter- 

 mine the changes that are induced in the animal organism 

 by a given variety of bacteria. 



In order to consider a bacterium as the cause of an in- 

 fective disease, in order to declare it specific for this disease, 

 it must comply with three conditions formulated by Koch : 

 In the first place, it must be present in all cases of the given 

 disease ; in the second place, it must occur only in this dis- 

 ease ; and, in the third place, it must be capable of induc- 

 ing essentially the same disease in experiments on animals. 

 Animal experimentation, therefore, plays a most important 

 role in bacteriology. 



In order to infect animals with bacteria various ways 

 are open. The natural portals of infection may be em- 

 ployed, and, besides, new ones may be artificially estab- 

 lished through which the microbes are introduced into the 

 organism. 



(a) Cutaneous Inoculation. Quite superficial wounds of 

 the skin are made in animals (as in vaccination), and these 

 are smeared, by means of the platinum wire, with a small 

 amount of pure culture. In guinea-pigs and in mice, in- 

 stead of cutaneous inoculation, an incision is made with 

 scissors through the margin of the ear, and the injured 

 places are rubbed with the inoculating material. 



(B) Subcutaneous Inoculation. By means of a scalpel or 

 an inoculating needle, a pocket is formed in the subcuta- 

 neous connective tissue, into which the bacterial material is 

 introduced ; or the bacteria suspended in water or in bouillon 

 are introduced beneath the skin by means of a hypodermic 

 syringe. 



(<r) Intravenous Injection. By means of a hypodermic 

 syringe the infecting fluid is injected directly into a vein 

 that either lies quite superficially (as the marginal vein 

 of the rabbit) or is exposed by dissection. Entrance of 

 air into the vein may cause immediate death through air- 

 embolism. It is therefore necessary to exclude carefully 

 all air-bubbles from the system, and, further, to compress 

 the vein with a small cotton pad, and close the wound 

 immediately after removal of the needle. 



