270 THE INFECTIOUS DISEASES. 



Recently changes have been described in the intervertebral ganglia 

 and in the plexiform ganglia of the pneumogastric nerve, which, al- 

 though not limited to rabies, are yet so frequently present as to be ap- 

 parently of value in diagnosis. The lesions consist in degeneration or 

 atrophy or destruction of the ganglion cells with a proliferation of the 

 endothelial cells lining the capsule.' 



THE EXCITANT OF THE DISEASE. 



While there is reason for believing that hydrophobia is due to the 

 introduction into the body of some special form of micro-organism, and 

 while the recent researches of Pasteur and others have brought to light 

 many interesting and important facts regarding the general nature and 

 distribution in the body of the infectious agent, nothing is yet definitely 

 known about the particular organism which induces the disease. 



It is known that the infectious agent is in the saliva and salivary 

 glands of rabid animals, and that it may be present in the saliva of the 

 dog from three to five days before the symptoms of the disease appear. 

 It is not present in the blood, but seems to be especially concentrated in 

 the central nervous system and particularly in the medulla oblongata. It 

 is readily rendered inert by corrosive sublimate and other germicides. 

 It resists cold even to 20 C. but loses virulence after one hour's ex- 

 posure to 50 C. 



PREVENTIVE INOCULATION. 



Notwithstanding the total ignorance of the micro-organism concerned 

 in inciting hydrophobia, his genius in wise experiment enabled Pasteur 

 to establish a method for artificial immunization against the disease 

 which has proved most beneficent. 



After obtaining a virus of high and definite intensity, which was ac- 

 complished by a series of inoculations beneath the dura mater in rabbits of 

 portions of the spinal cords of rabid animals, it was found that by dry- 

 ing in the air, spinal cords of rabbits having definite and high virulence, 

 with due protection against aerial contamination, the virulence dimin- 

 ished day by day. With virus thus obtained of virulence ranging from 

 that which is practically inert to that of the utmost potency, it has been 

 found possible safely to accustom both animals and men to the presence 

 of amounts of hydrophobia virus contained in the spinal cord emulsion, 

 which under ordinary conditions would prove speedily fatal. In other 

 words, it has been found possible to confer artificial inimunitj against 

 the disease. 



This process occupies several days, and immunization must be com- 

 pleted before the disease has begun to manifest itself ; but as the incuba- 

 tion period in hydrophobia is a long one the average is about forty days 

 it has been possible, in a large and increasing number of cases, to save 

 the lives of persons bitten by rabid animals. 



1 See Rmenel and McCarthy, University Medical Magazine, vol. xiii. p. 766, 1901. 



