SYPHILIS IN ANIMALS. 649 



with positive results, when the higher types of 

 monkeys were used. Confirmation has come from 

 a number of independent experimenters (e. g., 

 Lassar, A. ISTeisser, Kraus, Flexner), and A. Neis- 

 ser in particular has taken up the work on an 

 extensive scale. 



Some of Neisser's work is of the utmost impor- J 

 tance. The experiments of Metchnikoff and Koux 

 had already indicated that the higher monkeys 

 (chimpanzee, etc.) acquired generalized syphilis 

 more readily than the lower species. Neisser's 

 work corroborates this, and he recognizes a scale of 

 susceptibility which corresponds roughly with the 

 proximity of the different species to man, as indi- 

 cated by general morphology and the reaction of 

 serum precipitins. The chimpanzee, orang-utan 

 and gorilla are the most susceptible, and the syph- 

 ilis produced in them approaches closely that seen 

 in man, including the secondary symptoms. It is 

 suspected that the cynocephalus varieties are less, 

 and the macacus varieties least susceptible. Among 

 the macaci the smaller types (rhesus) are more 

 resistant than the larger. The lower susceptibility 

 of these animals is recognized by the failure of 

 secondary symptoms to develop, hence in them the 

 syphilis may be purely local (Neisser). Spiro- 

 chetes have been found in all the lesions of experi- 

 mental syphilis in monkeys. 



Bertarelli first succeeded in producing experi- syphilis 

 mental syphilitic keratitis in the rabbit and found 

 associated with it the Spirochcuta pallida. His 

 work has been verified by various observers. Miih- 

 lens and others have been able to produce a pri- 

 mary lesion in the guinea-pig by material taken 

 from syphilitic keratitis in the rabbit. 



