INOCULATION OF ANIMALS. 127 



used must be made up and neutralised by the precise methods already 

 described. The investigator must give every detail of the methods 

 he has employed in order that his observations may be capable of 

 repetition. 



INOCULATION OF ANIMALS. 1 



The animals generally chosen for inoculation are the 

 mouse, the rat, the guinea-pig, the rabbit, and the pigeon. 

 Great caution must be shown in drawing conclusions from 

 isolated experiments on rabbits, as these animals often 

 manifest exceptional symptoms, and are very easily killed. 

 Dogs are, as a rule, rather insusceptible to microbic disease, 

 and the larger animals are too expensive for ordinary labora- 

 tory purposes. In the case of the mouse and rat the variety 

 must be carefully noted, as there are differences in suscepti- 

 bility between the wild and tame varieties, and between the 

 white and brown varieties of the latter. In the case of the 

 wild varieties, these must be kept in the laboratory for a 

 week or two before use, as in captivity they are apt to die 

 from very slight causes, and, further, each individual should 

 be kept in a separate cage, as they show great tendencies to 

 cannibalism. Of all the ordinary animals the most suscep- 

 tible to microbic disease is the guinea-pig. Practically all 

 inoculations are performed by means of the hypodermic 

 syringe. The best variety is made on the ordinary model 

 with metal mountings, asbestos washers, and preferably 

 furnished with platinum iridium needles. Before use the 

 needle is mounted on the piston and the syringe sterilised 

 by boiling for five minutes. The materials used for inocu- 

 lation are cultures, animal exudations, or the juice of organs. 

 If the bacteria already exist in a fluid there is no difficulty. 

 The needle is most conveniently filled out of a shallow 

 conical test glass which ought previously to have been 

 covered with a cover of filter paper and sterilised. If an 

 inoculation is to be made from organisms growing on the 

 surface of a solid medium, either a little ought to be scraped 



1 Experiments on animals, of course, cannot be performed in this 

 country without a license granted by the Home Secretary. 



