EXPERIMENTS UPON ANIMALS. 99 



ture medium. It is also an essential point when pathogenic bacteria 

 are injected which kill susceptible animals in very minute doses, for 

 it has been shown by the experiments of Watson Cheyne and others 

 that in the case of some of these, at least, there is a limit below 

 which infection does not occur. 



Inoculated animals should be carefully observed, and a note 

 made of every symptom indicating a departure from the usual con- 

 dition of health, such as fever, less of activity, loss of appetite, 

 weakness, emaciation, diarrhoea, convulsions, dilated pupils, the for- 

 mation of an abscess or a diffuse cellulitis extending from the point 

 of inoculation, etc. The temperature is usually taken in the rectum. 

 The temperature of small animals, like rabbits and guinea-pigs, va- 

 ries considerably as a result of external conditions. In the rabbit 

 the normal temperature may be given as about 102 to 103 F, ; in 

 the guinea-pig it is a little lower. 



In making a post-mortem examination of an inoculated animal it 

 is best to stretch it out on a board, belly up, by tying its legs to nails 

 or screws fastened in the margin of the board. When the abdomen 

 is dirty, as is usually the case, it should be carefully washed with a 

 disinfecting solution. An incision through the skin is then made in 

 the median line the full length of the body, and the skin is dis- 

 sected back so as to expose the anterior walls of the abdomen and 

 thorax. These cavities are then carefully opened with a sterilized 

 knife or scissors, and the various organs and viscera examined. At- 

 tention should also be given to the appearances at the point of in- 

 oculation. To ascertain whether the microorganism injected has 

 invaded the blood, smear preparations should be made with blood 

 obtained from a vein or from one of the cavities of the heart. It 

 will be well also to make a smear preparation from a cut surface of 

 the liver and spleen. In the various forms of acute septicaemia the 

 spleen is usually found to be enlarged. If but few microorganisms 

 are present in the blood and tissues they may escape observation in 

 stained smear preparations, and it will be necessary to make cultures 

 to demonstrate their presence. A little blood from a vein or from 

 one of the cavities of the heart is transferred, by means of a plati- 

 num loop (ose) or a sterilized collecting tube (see page 38), to a 

 test tube containing liquefied nutrient gelatin or agar-agar, and an 

 Esmarch roll tube is made. This is put aside for the development of 

 colonies from any scattered bacteria which may be present. As a 

 rule, it will be best to make agar cultures, as these can be placed in 

 the incubating oven at 35 to 38 C. Stick^cultures may also be 

 made and will serve to show the presence of microorganisms, but 

 will not give information as to how numerous they may be. The 

 roll tube also has the advantage of showing whether there is a 



