514 ANNUAL REPORT OF THE Off. Dc»:. 



for two days before the appearance of symptoms. The symptoms 

 do Dot show themselves until the poison or virus has remained in 

 the nervous tissues long enough to bring about changes in their 

 structure and functions. 



Method of Invasion. When introduced into another animal either 

 experimentally or in the natural course of the disease, the virus 

 remains for a time without producing either local or general symp- 

 toms, undergoing a period of incubation during which it undoubt- 

 edly multiplies itself, in this respect corresponding to the well 

 known infectious diseases. It may be removed from the saliva by 

 filtration through porcelain, proving that it is a solid body. The 

 ^■irus penetrates to the nervous system by following the nerve 

 trunks from the site of injury to the spinal cord, then the spinal 

 cord to the medulla and brain. This has been proven by inoculating 

 an animal in one of its legs with virulent material. After a suit- 

 able time, but before the symptoms of rabies appear, if the animal 

 be killed, the virus will be found in the nerves of the limb, and even 

 in the part of the spinal cord into which the nerves enter, while the 

 upper part of the cord and brain will be free from it. This fact 

 explains the reason why the earliest symptoms, both in man and 

 animals, such as pain, itching, tingling, numbness and other nerv- 

 ous sensations, often appear in the part of the body which received 

 the virus through the bite. In the case of a bite about the face 

 and head, the route along the nerves to the central nervous system 

 is shorter still. While the nerves then form the main route by 

 which the virus travels, the circulation may at times assist, especi- 

 ally in small animals. Inoculation into the large nerve of the leg 

 is almost as certain to produce the disease, as inoculation directly 

 into the sub-dural space, while injection beneath the skin of the 

 leg is not so sure. 



Resistance OF Virus. The action of the virus is destroyed by dry- 

 ing, and by the action of light. In dry air protected from the light 

 and from putrefaction, the virulence of the spinal cord of rabbits 

 is destroyed in fourteen to fifteen days. When spread in thin layers 

 the virus is destroyed entirely by drying in four to five days. Sun- 

 light destroys it in about forty hours. The loss of virulence by dry- 

 ing is gradual and quite regular, iind this is taken advantage of in 

 the preparation of the 'S^accine," which is described later. The 

 virus may be preserved unchanged in neutral glycerine at ordinary 

 temperature for a long time. Roux found that after four weeks 

 in glycerine at 30 degrees C, the virus has the same power as 

 when perfectly fresh. 



It is quite resistant to putrefaction. Galtier has found the virus 

 active in flu- ...nfrnl nervous system of rabbits buried for twenty- 



