3IO RABIES 



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

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

 rabbits is destroyed in fourteen to fifteen days. When spread 

 in thin layers it is entirel}- destroyed by drying in from four to 

 five days. Sunlight destroys it in about fort}- hours. The 

 loss of virulence by drying is gradual but quite regular, which 

 fact was taken advantage of by Pasteur in the preparation of his 

 vaccine. The virus may be preserved in neutral glycerin at 

 ordinary temperature for a long time. Roux found that after 

 four weeks in glycerin at 30° C, the virus in a rabid brain 

 had the same power as w^hen perfecth- fresh. The writer has 

 found that rabbits inoculated with rabid brains that had been 

 kept in glycerin from three to four weeks did not develop the 

 disease as quickh' as when they were inoculated with the 

 freshly removed brain. 



It is quite resistant to putrefaction. Galtier has found 

 the virus active in the central nervous system of rabbits buried 

 for twenty-three days, of sheep buried thirt3'-one days and of 

 dogs buried forty-four days. Other observers have found it 

 still active in animals buried for twenty-four days. 



It is destroyed completely by a temperature of 50° C. in 

 one hour or 60° C. in one-half hour. It is uninjured by 

 exposure to extreme cold, resisting the prolonged application 

 of a temperature from 10 to 20° C. below zero. 



Its activity is de.stroyed in one hour b}- a five per cent 

 solution of carbolic acid, or by a i to 1,000 solution of corro- 

 sive sublimate. Water saturated with iodine destro}-es it in 

 ten minutes. 



§ 239. Period of incubation. The period of incuba- 

 tion is quite variable depending on the site of the wound 

 which is almost always a bite, the amount of virus introduced 

 and its virulence. In general it may be said for all animals 

 that the period of incubation seldom exceeds sixty days, 

 though in man and in some of the larger animals, it some- 

 times, though very rarely reaches one year. A few cases of a 

 longer period have been reported. The average period as 

 given b}- Ravenel is as follows : 



In man, 40 da^'S. 



