258 THE FILTERABLE VIRUSES 



They are found particularly in the cornu of Ammon; in Purkinje's 

 cells in the cerebellum; and in the larger cells of the cortex of the 

 cerebrum. These may be the cause of the disease, but there are 

 several objections to this hypothesis. Their distribution does not 

 correspond to the parts of the nervous system that are most 

 intensely affected in hydrophobia, i.e., medulla and pons. In the 

 latter locality these bodies are rarely encountered. They are not 

 found invariably in animals dead from rabies, and are considered 

 to be too large to pass through a Berkefeld filter; this latter view 

 may not be a correct one. The finding of these bodies has been 

 considered by Negri to be good grounds for considering the case 

 to be hydrophobia. The rapid diagnosis of the disease in animals 

 can only be effected by killing them and examining the nervous 

 tissues, or inoculating other animals with them. Histologically, 

 three marked changes may be noted: (i) The finding of the Negri 

 bodies. (2) The finding of the degeneration of the cells of the 

 larger ganglia with the proliferation of the endothelial cells lining 

 the ganglionic spaces (Van Gehutchen and Nelis). (3) The find- 

 ing of certain tubercles in the medulla, which are called Babes 

 tubercles, though these are not wholly characteristic, as they are 

 found in other diseases. Hydrophobia is transmitted from the 

 site of the wound to the central nervous tissues by the nerves, and 

 the incubation period varies with the distance of the wound from 

 the central nervous system; the majority of cases occur between 

 twenty to sixty days after a bite. 



Immunity against infection and the development of the disease 

 after the reception of an infected wound, may be accomplished by 

 Pasteur's method (see chapter on Vaccine). 



Yellow Fever 



That this disease is caused by a parasite there can be no doubt. 

 It is highly infectious and largely confined to the tropical regions' 

 of the western hemisphere and in parts of Africa. Resembling 

 diseases established as due to protozoa, this one is unquestion- 



