ETIOLOGY 465 



bers in every case of a disease, and only in that disease, 

 appearing at the time the host tissue becomes infective in 

 regions that are infective, and increasing in these infective, 

 areas with the course of the disease, can be no other, accord- 

 ing to our present views, than the cause of that disease. 



The Negri bodies or cell inclusions vary in shape. The 

 most common forms are round or oval. The round bodies are 

 from .5 to 23/< in diameter, while the oblong ones vary from 

 .5 to 1.5// to 5 to 27/<. The round or oval forms are by far the 

 most common. Irregular forms are occasionally seen. In 

 preparations stained with eosin and methylene blue they ap- 

 pear as bright red bodies containing one or two nuclear-like 

 structures which are surrounded by a number of other small 

 circular, regular bodies. They are said to preserve their form 

 even when the brain tissue has undergone marked degenera- 

 tive changes, after prolonged immersion in glycerin and after 

 several days' drying. A number of workers report finding 

 them quite uniformly. Schiider does not consider them as 

 the cause, largely because his virus passed through a filter that 

 retained the cholera spirillum. 



Remlinger and Riffat-Bey found that the virus would 

 pass through a Berkefeld filter "V," but its passage was not 

 constant, as rabbits inoculated with the filtrate did not all die 

 of rabies. The filter which he used held back the organism 

 of chicken cholera, which was used as a check on the filter. 

 Remlinger more recently stated that the virus will pass through 

 the more porous Berkefeld filter only. The firmer Berkefeld 

 and Chamberland filters keep the virus back. 



Babes states that the Negri bodies are not always present. 

 He seems to consider them as a result of the local reaction of 

 the cells to the cause of the disease. 



i- 370. Method of invasion. When introduced into an 

 animal either experimentally or by the bite of a rabid dog, the 

 virus remains for a time without producing either local or 

 general symptoms. The period of incubation varies withm 

 quite wide limits. The virus penetrates the nervous system 

 by following the nerve trunks from the site of infection to the 



