EXPERIMENTS UPON ANIMALS. 101 
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, lcss of activity, loss of appetite, 
weakness, emaciation, diarrhcea, 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 microdrganism 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 septicemia the 
spleen is usually found to be enlarged. If but few microdrganisms 
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 (6se) 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. Stab cultures may also be 
made and will serve to show the presence of microdrganisms, 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 
