INOCULATION OF ANIMALS 143 



by Gram, yield only moderate surface growths, form acid in 

 carbohydrates, and produce no pigment or a white or orange 

 colour. The latter come chiefly from air and water, often are 

 sarciniform, decolorise by Gram, grow well on the surface of 

 media, do not ferment carbohydrates, and produce red or yellow 

 pigment. On similar lines, further subdivision of the groups 

 could be effected. It is manifest that important means of 

 differentiating allied bacteria may be available by the extended 

 application of this method. 



INOCULATION OF ANIMALS.! 



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 made on the ordinary model with metal mountings, 

 asbestos washers, and 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 a cover of filter paper and 

 sterilised. If an inoculation is to be made from organisms grow- 

 ing 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 



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

 out a licence granted by the Home Secretary. 



