SPREAD OF VIRUS IN ORGANISM OF SUSCEPTIBLE ANIMALS 611 



the ganglion cells of the central nervous systems of higher animals, 

 they furnish one of the most wonderful examples of how a parasite 

 in the struggle for existence, with the survival of the fittest, may 

 develop those properties which will insure its propagation from one 

 host to another. To the parasitic tubercle bacillus are open a thousand 

 and one routes by which it may be transferred from one animal to 

 another; to the neuroryctes hydrophobise, as far as known, only one 

 way, namely, that of an infected animal biting and wounding another. 

 There is no disease which, particularly in carnivora, leads to such an 

 intense, insane desire to bite as does hydrophobia, and this symptom 

 must be looked upon as one due to disturbances in the ganglion 

 cells depending upon poisons developed and produced by the neuro- 

 ryctes for the sole and special purpose of insuring the survival of 

 the race after the death of the host, which it kills by its own multi- 

 plication. That the parasitic microorganisms of rabies should produce 

 as one of the very prominent symptoms of the disease the psychic 

 disturbances which lead to the intense desire to bite, particularly 

 in canines, cats, rats, and occasionally in rabbits, is the more remark- 

 able when it is considered that the tetanus toxin which also directly 

 affects the central nervous system travels to it along the peripheral 

 nerves, produces symptoms, as far as convulsions and paralyses are 

 concerned, very similar to the rabic virus, does not in any way lead 

 to similar or identical psychic disturbances. Tetanus, however, 

 cannot be spread through the bite of an infected animal and the 

 generally saprophytic tetanus bacillus does not depend for the pre- 

 servation of the race upon the invasion of higher animals. 



Another striking feature in the life history of neuroryctes hydro- 

 phobise is the fact that while not to any extent found in other tissues 

 but those of the central nervous system, it is always present in the 

 salivary glands and excreted with the saliva to thus insure its transfer 

 to a new host. The mode of transfer from one host to another is 

 identical in principle with that found in malaria. However, in this 

 disease the protozoan parasites are first taken up by an intermediary 

 host, the mosquito, to wander from its stomach into the salivary 

 glands, and to be transferred by biting to its more permanent host, 

 man, birds, or other animals. 



Spread of the Virus in the Organism of Susceptible Animals. It has 

 been shown by the experiments of Pasteur and others that the rabic 

 virus from its place of entrance in the body of a susceptible animal 

 travels along the peripheral nerves to the central nervous system. 

 Rabies can always be produced by injecting the virus directly into 

 the peripheral nerves or even by moistening with an emulsion the 

 central end of a divided nerve. On the other hand, if the peripheral 

 end of a cut nerve is infected and the connection between this part 

 and the central nervous system carefully destroyed then infection 

 does not occur. The most pronounced changes in the cord in rabies 

 are always found in that part which supplies the peripheral nerves 



