DISEASES OF UNKNOWN ETIOLOGY 651 
central nervous system, um\^ the ^ray substance as far as possible, 
is injected subdurally into an experimental animal for a final diag- 
nosis. The method of animal inoculation, while slower than the 
microscopic examination of the brain, is the final test in doubtful 
cases. Of course, treatment should not await the results of animal 
inoculation if there is suspicion that a patient has been bitten by a 
rabid dog, especially if the hands, face or other iniprotected surface 
be the site of the wounds 
Staining Negri Bodies.— Williams and Lowden' have developed a 
techni(iue for the raj)id demonstration of Negri bodies, which is widely 
followed at the present time. A small piece of the gray substance 
from the region of the hippocampus major and from the cerebellum 
of the animal is placed upon a clean glass slide and covered with a 
clean cover-glass. Pressure is applied to the latter until the tissue is 
flattened and spread uniformly. The pressure is now shifted to one 
edge of the cover-glass and the flattened tissue is forced along the 
slide, leaving a thin film as it passes. Fixation with neutral absolute 
methyl alcohol (Merck reagent) containing about 0.1 per cent ])icric 
acid (about ten minutes are required) is followed by removal of the 
fixing agent with filter paper. 
A small amount of a freshly prepared staining mixture, made in 
the proportions of 30 cc. of distilled water, 10 cc. of a saturated alco- 
holic solution of methylene blue and 0.5 cc. of a saturated alcoholic 
solution of basic fuchsin is poured over the slide, warmed until steam 
arises, then poured oft'. The excess stain is removed in running water 
and the preparation is carefully dried with filter paper. The prepara- 
tion is examined with an oil immersion lens. 
Negri bodies, which vary in size from about 1 micron to 25 microns 
in diameter, are stained magenta with blue granules by this process; 
the cytoplasm of the nerve cells is pale })lue; the nuclei of the nerve 
cells are colored a darker blue; a simpler method, fully as satisfactory, 
is that of Harris.'- 
The Pasteur Treatment for Rabies. — Pasteur^ made the very important 
observation that the \irus of rabies as it exists in rabid dogs (street 
virus) could be so attenuated by repeated passages through rabbits 
that it lost much of its original virulence for the dog. This change in 
virulence was fully established when passage of the virus from rabbit 
to rabbit caused each successive animal to sicken in about six or seven 
days, and to die regularly on the ninth day. No further increase in 
pathogenicity for the rabbit could be induced, and the virus at this 
level of virulence was called "virus fixe" by Pasteur. The spinal cord 
of such a rabbit, dried for two weeks o^•er caustic soda at room tem- 
perature, lost its virulence for rabbits, although cords dried for a week 
or ten days killed the animal when injected subdurally; the period 
of incubation was, of course, increased when the partly dried cords 
were used. 
' Jour. Infec. Dis., 1906, 3, 452. ^ Il)id., 1908, 5, 5G6. ' Lor. cil. 
