VARIATIONS IN VIRULENCE OF THE BACILLUS. 369 



that this paresis is due to well-marked changes in the nerves. 

 The medullary sheaths first become affected, breaking up into 

 globules ; ultimately the axis cylinders are involved, and may 

 break across, so that degeneration occurs in the peripheral 

 portion of the nerve fibres. Such changes occur irregularly in 

 patches, both sensory and motor fibres being affected. Fatty 

 change takes place in the associated muscle fibres. There may 

 also be a similar condition in the cardiac muscle. The organic 

 acid has a similar but weaker action. Substances obtained from 

 diphtheria membrane have an action like that of the bodies 

 obtained from the spleen, but in higher degree. Martin con- 

 siders that this is due to the presence in the membrane of an 

 enzyme which has a proteolytic action within the body, resulting 

 in the formation of poisonous albumoses. According to this 

 view the actual toxic bodies are not the direct product of the 

 bacillus, but are formed by the enzyme which is produced by it 

 locally in the membrane. Cartwright Wood has also found that 

 when diphtheria cultures in an albumin-containing medium are 

 filtered germ-free and exposed to 65° C. for an hour (the supposed 

 ferments being thus destroyed), there still remain albumoses 

 which produce febrile reaction and are active in developing 

 immunity. The existence of ferments, though a possibiUty, 

 cannot, however, be considered to be yet completely proved. 

 Nor is it yet certain whether the proteids obtained by precipita- 

 tion from cultures and from the tissues are in themselves toxic, 

 or whether the toxic bodies are carried down along with them. 



Immunity. — This is described in the general chapter on 

 Immunity. It is sufficient to state here that a high degree of 

 immunity, against both the bacilli and their toxins, can be pro- 

 duced in various animals by gradually increasing doses either 

 of the bacilli or of their filtered toxins {vide Chapter XX.). 



Variations in the Virulence of the Diphtheria Bacillus. — In 

 cultures on serum the diphtheria bacilli retain their virulence 

 fairly well, but they lose it much more quickly on less suitable 

 media, such as glycerin agar. Roux and Yersin found that, 

 when the bacilli were grown at an abnormally high temperature, 

 namely, 39.5° C, and in a current of air, the virulence diminished 

 so much that they became practically innocuous. When the 

 virulence was much diminished, these observers found that it 

 could be restored if the bacilli were inoculated into animals along 



