METHOD OF INVASION 309 



taiti instances its presence has been reported in salivar}- glands, 

 the pancreas and milk. It has not been found in the blood. 

 A very large amount of work has been done by way of investi- 

 gation to determine, if possible, the nature of the specific 

 cause but thus far positive results have not been recorded. It 

 should be stated, however, that a number of workers have 

 found in the brain or cord of affected animals various forms of 

 bacteria and Blastomycetes but in every instance the conclusions 

 reached by the authors have not been confirmed by others. 

 While it is true that the cause has not been recognized, the 

 researches in recent times have brought to our knowledge 

 many facts concerning it. It can be removed from the saliva 

 by filtration which indicates that it is a solid particle. 



§ 238. Method of invasion. When introduced into an 

 animal either experimentally or bv the bite of a rabid dog, the 

 virus remains for a time without producing either local or 

 general symptoms. The period of incubation varies within 

 quite wide limits. The virus penetrates the nervous system 

 by following the nerve trunks from the site of infection to the 

 spinal cord, then through the spinal cord to the brain. This 

 has been proven by inoculating an animal in one of its legs 

 with virulent material. After a suitable time, but before the 

 symptoms of rabies appear, the virus will be found, on killing 

 the animal, in the nerves of the limb, and even in the part of 

 the spinal cord into which the nerve enters, while the upper 

 part of the cord and the brain are still uninfected. This ex- 

 plains the fact why the earliest symptoms, both in man and ani- 

 mals, such as itching, tingling, numbness and other nervous 

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

 the virus. In the case of a bite about the face and head the 

 route along the nerve to the central nervous system is much 

 shorter. While the nerves seem to form the main route by 

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

 especially in small animals. Inoculation into the large nerve 

 of the leg is almost as certain to produce the disease, as inocu- 

 lation directly into the sub-dural space, while injection beneath 

 the skin of the leg is not so sure. 



ResistMice of the virus. The action of the virus is destroyed 



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