THE INOCULATION OF ANIMALS. 237 
action in the animal body both without and under the 
influence of specific serums upon them. In this way 
the typhoid and colon bacilli may be separated, or the 
pneumococcus and streptococcus. Still, a different use 
of animals is to measure the protective effect of anti- 
toxic and bactericidal serums; thus, diphtheria antitoxin 
is added to diphtheria toxin and injected into guinea- 
pigs, and streptococcus immunizing serum is mixed 
with living streptococci and injected into the vein of 
a rabbit. The use of animals to develop through bac- 
terial injections protective serums will be dealt with 
under the special bacteria by whose products they are 
produced, 
THE INOCULATION OF ANIMALS. 
The inoculation of animals may be made either 
through natural channels or through artificial ones: 
1. Cutaneous. Cultures are rubbed into the abraded 
skin. 
2. Subcutaneous. The bacteria are injected by 
means of a hypodermatic needle under the skin, or 
are introduced by a platinum loop into a pocket made 
by an incision. 
3. Intravenous. The bacteria are injected by means 
of a hypodermatic needle into the vein. This is usually 
carried out in the ear vein of the rabbit. If rabbits 
are placed in a holder, so that the rabbit remains quiet 
and only the head projects, it is usually easy to pass a 
small needle directly into one of the ear veins, espe- 
cially those running along their edges. If the ear is 
first moistened with a 3 per cent. carbolic acid solution, 
and then supported between the finger inside and the 
