INOCULATION OF ANIMALS. 117 



the actual sizes can easily be calculated. In describing bacterial cultures it 

 must be borne in mind that the appearances often vary with the age. It is 

 suggested that in the case of cultures grown at from 36 to 37 C. the appear- 

 ances 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 investiga- 

 tor must give every detail of the methods he has employed in order that his 

 observations may be capable of repetition. 



INOCULATION OF ANIMALS 



The animals generally chosen for inoculation are the mouse, 

 the rat, the guinea-pig, the rabbit, and the pigeon. Great cau- 

 tion must be shown in drawing conclusions from isolated experi- 

 ments 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 labo- 

 ratory 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 tenden- 

 cies to cannibalism. Of all the ordinary animals the most sus- 

 ceptible 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 mount- 

 ings, asbestos washers, and preferably furnished with platinum 

 indium 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 75 per cent salt solution to make an emulsion, 

 or a little sterile fluid is poured on the growth and the latter 



