GENERAL CONSIDERATION OF FILTRABLE VIRUS 901 



about the head, neck, and upper extremities and are deeply lacerated. 

 This is explained by the fact that the poison is conveyed to the 

 central nervous system chiefly by the path of the nerve trunks. 

 This has been experimentally shown by di Vcstca and Zagari 18 who 

 inoculated animals by injection into peripheral nerves, and showed 

 that the nerve tissue near the point of inoculation becomes infectious 

 more quickly than the parts higher up; thus the lumbar cord of 

 an animal inoculated in the sciatic nerve is infectious several days 

 before virus can be demonstrated in the medulla. 



In man, infected with "street virus," that is, with the virus of 

 a dog or other animal not experimentally inoculated, the incubation 

 period varies from about forty to sixty days. Isolated cases have 

 been reported in which this period was prolonged for several months 

 beyond this. 



The virulence of rabic virus may be markedly increased or 

 diminished by a number of methods. By repeated passage of the 

 virus through rabbits, Pasteur 19 was able to increase its virulence 

 to a more or less constant maximum. Such virus which had been 

 brought to the highest obtainable virulence, he designated as "virus 

 fixe." Inoculation of rabbits, dogs, guinea-pigs, rats, and mice with 

 such virus usually results in symptoms within six to eight days. 

 The same animals inoculated with street virus may remain ap- 

 parently healthy for two to three weeks. 



In dogs and guinea-pigs inoculation usually results first in a stage 

 of increased excitability, restlessness, and sometimes viciousness. 

 This is followed by depression, torpor, loss of appetite, inability 

 to swallow, and finally paralysis. In rabbits the disease usually 

 takes the form of what is known as "dumb rabies," the animals 

 gradually growing more somnolent and weak, with tremors and 

 gradual paralysis beginning in the hind legs. 



In experimentally infected birds the disease is slow in appearing 

 and may show a course of gradually increasing weakness and 

 progressive paralysis extending over a period of two weeks after 

 the appearance of the first symptoms. 



In man, the disease begins usually with headaches and nervous 

 depression. This is followed by difficulty in swallowing and spasms 

 of the respiratory muscles. These symptoms occur intermittently, 



18 di Vested and Zagari, Ann. de 1'inst. Pasteur, iii. 



19 Pasteur's work on rabies. Compt. rend, de 1'acad. des sciences, 1881, 1882, 

 1884, 1885, 1886, and Ann. de 1'inst. Pasteur, 1887 and 1888. 



