204 ESSENTIALS OF VETERINARY LAW 
in the case of serum supplied by many states. It 
is well to use serum from two or more lots at the 
same time, and when this is practiced they should 
not be used indiscriminately. When so used it 
would be practically impossible to trace the in- 
fection should harm result. On the other hand, 
a ‘‘control’’ test may offer efficient aid. For exam- 
ple: Suppose a veterinarian has lots of X and Y 
serum, enough to treat herds of A. and B. If he 
used the lots of serum indiscriminately, and 
disease broke out soon after, there would be little 
to show that the disease was due to the use of 
either lot. If X were used only on the A herd, 
and Y on the B hogs, the occurrence of foot and 
mouth disease in the A herd would point to the X 
serum as a possible source. But if X were used 
on the A herd except two or three animals, on 
which Y was used; and Y were used on the B herd, 
except a few on which X were used; then if the 
disease occurred only among the A herd, irrespect- 
ive of the serum used, it would indicate that the 
disease was not due to the serum; but if it showed 
first in the X animals of each herd it would be 
strong evidence of infection in the X serum. In 
such use of two lots the exceptional animals in 
each herd must be clearly and surely indicated in 
the record. 
If in addition the veterinarian made a practice 
of reserving one package from each lot used, for 
a period of from fifteen to thirty days, there would 
be a possibility of surely demonstrating the qual- 
ity of the article. There is another, and a selfish 
reason for the veterinarian adopting this practice. 
If after the use of serum, septicaemia breaks out 
