120 INOCULATION OF ANIMALS 



often vary with the age. It is suggested that in the case of cultures 

 grown at from 36 to 37 C. the appearances between 24 and 48 hours 

 should be made the basis of description, and in the case of cultures 

 grown between 18 and 22 C. the appearances between 48 and 72 hours 

 should be employed. The culture fluids 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. l 



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 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 platmum-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 growing on the surface 

 of a solid medium, either a little ought to be scraped off and 

 shaken up in sterile distilled water 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 filtered into 

 the test-glass through a plug of sterile glass wool. This is easily 

 effected by taking a piece of f in. glass-tubing 3 in. long, drawing 



1 Experiments on animals, of course, caunot be performed in this country 

 without a license granted by the Home Secretary. 



