Page:Popular Science Monthly Volume 28.djvu/302

290 This method is based essentially upon the following facts:

Inoculation of a rabbit, by trepanning, under the dura mater, with the poisonous marrow of a mad dog, always gives rabies to the animal after a mean period of incubation of about fifteen days. If the virus is passed from this first rabbit to a second, from this one to a third, and so on, by the same method of inoculation, there is shortly manifested a more and more marked tendency toward a shortening of the period of incubation in the rabbits successively inoculated. After from twenty to twenty-five passages from rabbit to rabbit, we arrive at a period of incubation of eight days, which is maintained during a new series of from twenty to twenty-five passages. Then we have a period of incubation of seven days, which occurs with striking regularity during a new series of passages rising to the ninetieth. At least that is the number I have now reached without having hardly yet observed a tendency to a slight further shortening of the period.

The experiments of this character, which I began in November, 1882, have already been continued for three years without the series having been interrupted, or without my having used any other virus than that from rabbits which successively died rabid. Nothing, therefore, is more easy than to have at one's disposition, during considerable intervals of time, a virus of perfect purity, always identical, or nearly so. This is the practical point of the method.

The marrows of these rabbits are infected with rabies of constant virulence in their whole extent. If we detach from them pieces a few centimetres long, taking the greatest possible precautions to insure their purity, and suspend them in dry air, the virulence of the rabies in them will slowly pass away, till it is quite extinguished. The duration of the process varies somewhat with the thickness of the marrow, but depends chiefly on the exterior temperature: the lower the temperature the longer the virulence lasts. These results constitute the scientific point of the method.

These facts being substantiated, we have the following method of making a dog, within a reasonably short time, proof against rabies.

In a series of flasks, the air of which is kept dry by pieces of potash in the bottom, we suspend each day a piece of freshly infected marrow from a rabbit that has died of rabies, developed after seven days of incubation. Every day, at the same time, we inoculate under the skin of a dog a Pravaz syringeful of sterilized broth, in which has been soaked a small piece of one of the marrows we are keeping in desiccation, beginning with one of those which we have prepared several days before our operation is performed, so as to be sure that it is not of full strength. On that subject we have informed ourselves by previous experiments. We operate in the same manner on the following days