INOCULATION OF ANIMALS 137 



difficulties arise in consequence of the existence of groups of 

 organisms presenting closely allied characters, and the difficulty 

 and importance of identification is enhanced by the fact that 

 the same group may include both harmful and innocent members. 

 Examples of this occurrence are found in the pyogenic cocci and 

 their allies, in the coli-typhoid group of bacilli, and in the group 

 of cholera vibrios. In such cases it is usually necessary to take 

 into account all the morphological and cultural reactions of air 

 organism before it can be adequately classified. 



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 insus- 

 ceptible to microbic disease, and the larger animals are too 

 expensive for ordinary laboratory purposes. In the case of the 

 mouse and rat the variety must be carefully noted, as there are 

 differences in susceptibility 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 susceptible 

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

 are performed by means of the hypodermic syringe. The best 

 variety is that of the " Record " type, preferably furnished with 

 platinum-iridium needles. Before use, the syringe and the 

 needle are sterilised by boiling for five minutes. The materials 

 used for inoculation are cultures, animal exudations, or the juice 

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

 difficulty. The syringe is most conveniently filled out of a 

 shallow conical test-glass, which ought previously to have been 

 covered with 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 off and 

 shaken up in sterile bouillon or - 85 per cent, salt solution to 

 make an emulsion, or a little sterile fluid is poured on the 

 growth, and the latter scraped off into it. This fluid is then 



1 Experiments on animals, of course, cannot, in Britain, be performed with- 

 out a licence granted by the Home Secretary. 



