278 LABORATORY EXERCISES IN BACTERIOLOGY. 



as a rule we are satisfied with experimentation upon one or two species of the smaller 

 animals easily available for laboratory work, as frogs, hens, pigeons, mice, rats, guinea- 

 pigs, rabbits, and dogs. This part of the study is dictated in the third of Koch's 

 postulates relating to the study of pathogens and the proof of their causal relationship 

 to disease: "The bacterium in pure culture is to be inoculated into a susceptible animal 

 and its effects compared with and recognized as similar or identical to the known symptoms 

 of the originally diseased individual." 



The selection of animals susceptible to the influence of an organism is by no 

 means always easy or possible in the small range available to the laboratory worker. 

 In general, however, it may be said that frogs are well suited for the psychrophilic 

 bacteria, and the small warm-blooded animals for the mesophilic varieties. After 

 inoculation the animals are to be kept in as nearly normal relations and surroundings 

 as possible, and it is well always to place in the same conditions, but in separate con- 

 tainers, one or more similar but uninoculated animals as controls. The inoculations 

 may be made in one or other of a variety of ways. Sometimes a mere scratching 

 of the skin, as in vaccination, with the culture laid on the surface and well rubbed 

 in with a small spatula, is sufficient. Or a bit of thread moistened with a fluid culture 

 may be drawn through the skin after the manner of applying a seton. Usually, how- 

 ever, more definite operations are required, and are preferable from more or less cer- 

 tainly avoiding the entrance of the common bacteria of the skin as contaminating 

 and confusing influences, hypodermic, intravenous, and intraperitoneal inoculations 

 being the usual forms in the warm-blooded animals. 



Hypodermic Inoculations. Mice are best held by the end of the tail in a glass 

 jar, the tail being drawn out over the edge of the jar and the mouse held in the jar 

 by a board over the top of the latter so placed as to permit the exposure of the tissues 

 at the base of the tail only. Guinea-pigs and rabbits may be held by an assistant, 

 the two hind legs in one hand, the two fore legs in the other; or one of the various 

 mechanical animal holders (Fig. 68) may be employed. Dogs had best be fastened 

 for any important inoculation in one of the troughs commonly used for this purpose 

 in experimental work, although for inoculation with the syringe this is scarcely neces- 

 sary. Pigeons and hens should be held with one hand grasping the legs, tips of the 

 wings, and the tail, and the other holding the neck in extension. Mice and rats are 

 usually inoculated in the tissues about the base of the tail and along the lower end 

 of the spine; rabbits and guinea-pigs, in the tissues of one side of the thorax; pigeons 

 and hens, in the pectoral region; the culture being usually introduced into one of 

 the subcutaneous lymph-sacs when frogs are used as experiment animals. 



It is next to impossible to thoroughly sterilize the skin, but the hair or fur should 

 be removed by clippers and razor or the feathers plucked from a small area about 

 the site of the proposed inoculation and the surface well washed with soap and water 

 and rinsed with boiled water (or a pledget of cotton soaked with a 1 : 1000 solution 

 of corrosive sublimate applied closely to the surface for a few minutes, alcohol and 

 ether being afterward used for rinsing). The culture may be suspended in sterile 

 water or physiologic salt solution, or an active bouillon culture employed, when it is 

 best introduced beneath the skin by means of a syringe. Or a dry culture may be 

 inserted into a small pocket under the skin upon a stiff platinum needle. The hypo- 

 dermic syringe used should be so constructed that it may be thoroughly sterilized, 

 made of glass, metal, and rubber (as Koch's or Strohschein's). An excellent substitute 

 may be extemporized from a glass pipette or Sternberg's bulb with a hypodermic 

 needle attached to the tube by means of a rubber tube, the fluid being forced from 



