236 THE FILTERABLE VIRUSES 



about five microns. 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. Histo- 

 logically, 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 

 finding 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. 



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. 



Like several of the protozoon parasites, this one is unquestionably 

 spread by mosquitoes, and it has been definitely determined by 



