228 
FOOT AND MOUTH DISEASE. 
bodies (psorosperms), which were found abundantly in the 
flesh of animals dead of cattle-plague, and other malignant 
diseases, nothing of an abnormal kind has been observed. 
It may, therefore, be safely assumed that the meat of animals 
affected with mouth and foot disease is harmless, the more so 
that it always, unlike milk, undergoes some culinary process 
before being consumed. 
From experiments, which will be hereafter referred to, it 
appears that the contagium is in a most active form in the saliva, 
and it is remarkable that this fluid contains the fewest morbid 
products. Nevertheless, its contact with the interior of the 
mouth of a healthy animal quickly causes the development 
of the disease. There is no evidence of communication of 
infection by the skins or other parts of dead animals, and it 
is not certain, although very probable, that the gaseous 
emanations from diseased beasts are dangerous to those in 
the immediate neighbourhood. The spread of the infection 
along a certain line of country from one farm to another is 
supposed to indicate contamination of the atmosphere, but, 
in considering this mode of communication, the fact of 
animals often remaining unattacked within five hundred 
yards of diseased cattle must be taken into account. 
Period of Incubation . — According to the general acceptation 
of the term, the time of incubation of an infectious disease 
is the period which elapses between the introduction of the 
virus into the system and the manifestation of symptoms 
which indicate the existence of the disease, and in this ordi- 
nary sense the expression is now used. Some difficulty 
usually presents itself in reference to the determination of 
the precise moment of infection. It does not follow that 
healthy animals herded with diseased ones should at once 
become the subjects of the malady ; on the contrary, they 
may resist its influence for some time, and, therefore, con- 
clusions which are drawn from observations of the natural 
progress of an infectious disease are often erroneous, and 
always doubtful. Direct transmission of the virus into the 
system of a healthy animal furnishes the only reliable means 
of ascertaining with absolute certainty the moment of infec- 
tion, and tested in this manner, the period of incubation of 
eczema ranges from 36 to 48 hours. If the rise of tempera- 
ture be taken as a positive symptom, which it is not, the 
incubation stage will be much shortened, as this elevation of 
internal heat sometimes precedes by 12 hours the formation 
of vesicles in the feet or mouth. The extremely rapid action 
of the virus is evident from the fact that symptoms of fever 
are apparent in 30 hours after incubation, and the full 
