102 
ROBERT MEADE SMITH. 
Thus it has been proved that the virus of rabies always de¬ 
velops itself in the nervous system, in the brain, the spinal cord 
and nerves, and in the salivary glands, and never simultaneously 
invades every part. It may, for example, lix itself in the spinal 
cord and then attack the brain ; or one may find it in one or 
more parts of the brain, and not in others. The sole point in 
which the virus of rabies -is invariably localized in all cases of 
rabies, and all cases are invariably fatal, is in the medulla oblon¬ 
gata ; and if the virus extracted from this portion of the nervous 
system is inoculated on the surface of the brain of a dog, rabies 
is uniformly produced. These two facts were the starting point 
for the discovery of a means of preventing this disease, which 
never arises spontaneously; but by themselves they would be of 
minor importance had they not led to the discovery of a method 
by which the virus of rabies might be so attenuated as to permit 
of inoculation without producing dangerous symptoms, and at the 
same tim'e confer complete immunity to the disease. The great 
difficulty that was met with at the outset of these studies was to 
obtain some standard by which attenuation of the virus might be 
recognized; for, as is well known, after being bitten by a mad 
dog, there is nearly always the greatest, difference in the duration 
of the period of incubation before the disease becomes manifest. 
Pasteur, however, found that while this period of incubation was 
very variable, according to the different modes of inoculation, it 
was uniform when the virus was injected under the arachnoid 
membrane, the interference with the length of the incubation 
period after a bite or intravenous injection depending upon the 
quantity of the virus which reaches the brain, while after inocu¬ 
lation into the brain the incubation is inversely proportionate to 
the strength of the virus. 
Having thus established a means of recognizing degrees of 
virulence in the poison of rabies, the next step was to artificially 
produce such an attenuation of virus as to produce no dangerous 
symptoms, and yet confer immunity from the disease. 
Jenner was the first to propound the idea that the poison 
which used to be called “ grease ” in horses, but which we now 
describe as horse-pox, must be attenuated in virulence by being 
