536 A MANUAL OF BACTERIOLOGY 



and possibly may be as long as two years, or even more ; 

 the average seems to be about ten weeks. In the rabbit, 

 after inoculation from the dog, the incubation period is 

 about two to three weeks. 



The virus resides in the central nervous system, as was 

 shown by Pasteur. Inoculation with emulsions prepared 

 from the medulla and with the saliva conveys the disease, 

 but the filtered emulsions are usually inactive, and the 

 other tissues and fluids of the body, excepting the lacrimals 

 and suprarenals, are non-infective. 



Remlinger * has found that after very complete tritura- 

 tion the virus may pass through a porcelain filter. 



No micro-organism has been demonstrated with certainty 

 in rabies. Negri has described the constant presence of 

 structures the Negri bodies particularly in the grey 

 matter of the hippocampus major, which he regards as 

 protozoa. They are of varying size, apparently encap- 

 suled, taking a homogeneous purplish colour in smears 

 stained with eosin and methylene-blue, the smallest 

 spherical and structureless, larger ones with a central 

 granule or nucleus, the largest, round, ovoid or elongated, 

 containing several (as many as eight) granules (Fig. 65). 

 They occur abundantly in animals suffering from chronic 

 rabies, but in the acute type are scanty, though still to 

 be found ; in " fixed virus " (p. 538) they are very small. 

 So constantly are the Negri bodies present in rabies, and 

 absent in non-rabic conditions, that their presence or 

 absence forms a rapid and simple means of diagnosis. 2 



Inasmuch as the rabies virus is filterable, the view 

 taken by Prowazek of the nature of the Negri bodies is 

 that they represent the tissue reaction to invasion by the 

 parasite, the parasite being an extremely minute one 



1 Bull, de I'InsL Pasteur, iv, 1904, p. 342. 



2 See Williams and Lowden, Journ. Infect. Diseases, vol. iii, 1906, 

 p. 452. 



