SCABIES. 159 



Hertwig has been a careful observer on the subject of 

 mange and scab in animals, and in 1827 made experiments 

 on scab in sheep, to prove that the disease was alone commu- 

 nicated by means of acari. In 1835 he published his ac- 

 count and illustrations of the acarus of the horse, and twenty 

 years later, he spoke of the sarcoptes canis. In 1838 Heriug 

 published his very remarkable memoir on the acari of the 

 horse, ox, sheep, cat, and chamois ; also describing the sar- 

 coptes cynotis and hippopodos, from the suppurating internal 

 ear of dogs, and from canker or paronychia of the feet of horses. 



We shall not enter into the history of the zoological 

 classification of the parasites in question. Writers on natural 

 history have been puzzled as much as the pathologist by 

 these insects, and we leave our readers to consult other 

 works, to learn the views of Linnaeus, Latreille, and many 

 others. Gerlach speaks of them as belonging to the class 

 Arachnida, the order of mites, or acari, sub-order of crawl- 

 ing mites, family sarcoptes, and he has three genera a, 

 Sarcoptes, that burrow in the skin ; 6, Dermatodectes, that 

 simply bite, and hold on to the skin ; and c, Symbiotes, liv- 

 ing together in large numbers, and not piercing further than 

 the cuticle in search of food. The species belonging to each 

 genus are as follows : 

 A. Sarcoptes. B. Dermatodectes. C. Symbiotes. 



1. S. hominis. 1. D. equi. 1. S. equi. 



2. S. equi. 2. D. bovis. 2. S. bovis. 



3. S. suis. 3. D. ovis. (To this genus be- 



4. S. canis. longs also: 



5. S. cati. S. elephantis.) 



6. S. caniculi. 



[To this genus belong also : 

 S. rupicaprae (Hering.) 

 S. dromedarii (Gervais.)] 



