92 
Contagious or Epizootic Abortion in Cows. 
atmosphere exerts a restraining influence on the bacilli. Some 
oxygen, however, is required for their growth, and the necessary 
amount is obtained at a little distance beneath the surface by 
diffusion of the gas into the medium, with the result that the 
bacilli there begin to multiply. Beneath that depth the oxygen 
fails to penetrate and growth is therefore prevented. 
Although this mode of growth is of value for enabling one 
to distinguish the abortion bacillus from many other organisms, 
it is presented by a few other bacteria. Moreover, it has 
been found that abortion bacilli can be induced to grow quite 
readily on the surface of artificial media 
where they are freely exposed to the 
atmospheric oxygen. 
Susceptibility of different animals 
to infection . — Putting aside one or two 
doubtful observations, natural cases of 
contagious abortion have hitherto been 
encountered in the bovine species only. 
Outbreaks of abortion are not uncommon 
among ewes and mares, but no outbreak 
among these animals has been definitely 
proved to have been caused by the abor- 
tion bacillus. Indeed, one may go farther 
and say that the particular disease which 
is the common cause of abortion in cows 
seldom or never attacks any of the other 
domesticated animals. 
It is remarkable, however, that the 
abortion bacillus is capable of causing 
abortion in a large number of animals 
— a fact which has been ascertained by 
experiment, and which appears to be 
out of harmony with experience. Preg- 
nant animals belongingto all the common 
domesticated species have been success- 
fully infected by experiment with the abortion bacillus, as 
have also the rabbit and guinea-pig. Indeed no species of 
mammal which has hitherto been tested by experiment has been 
found to be immune against this organism. It is, however 
not necessary on this account to doubt the opinion stated above, 
viz., that other animals than those belonging to the bovine 
species seldom or never contract the disease by contagion or 
infection, for there are many other well-known instances of 
marked discrepancy between the disease -producing power of 
an organism as determined by experiment and the incidence 
of the disease which it causes naturally. For example, it is 
very easy to infect sheep experimentally with tuberculosis, but 
Fig. 2.— Artificial culture of 
abortion bacilli in serum- 
gelatin-agar. The growth is 
confined to the dark stratum 
extending across the tube a 
short distance below the 
surface of the medium. 
