704 DISEASES OF THE SKIN. 



and towards the frout, long stiff hairs. The females, which 

 are far more abundant, only differ from the males in 

 their size, being about one- third longer, and in the struc- 

 ture of the two posterior pairs of feet, which are attached 

 to the belly, close together, of equal length and strength, but 

 shorter and weaker than in the male. The external (third) 

 pair of feet has two long terminal hairs, but no sucking disc. 

 The internal (fourth) pair has the rudiments of a claw, a sucking 

 disc, and a fine terminal hair at the base of the stalk of the 

 sucking disc. The colour of the parasite is whitish ; the horny 

 skeleton reddish-brown — ferruginous. The eggs are very large, 

 coated with a sticky mass, and are often carried about by the 

 female between her legs. The eggs are deposited in small 

 galleries under the epidermis. Tlie young are very small, but 

 grow very quickly during the first four days ; and, according to 

 Hertwig, have their eight feet at once, but only six according 

 to Hering. Their movements are effected quickly, and by 

 means of the feet provided with sucking discs. They may be 

 kept alive for three weeks without nourishment. Wherever 

 they penetrate the skin, a small knotted elevation is produced, 

 with a small passage, at the extremity of which the acarus sits. 

 The epidermis becomes soft, separates from the true skin, and 

 an exudate is thrown out, which, along with the epidermis, 

 dries into scaly scurfs, that become detached. 



The transference of this acarus from the horse to man has 

 been proved by many observers — for example, E. Viborg, Sydow, 

 Osiander, Greve, Groguier, and Hertwig — as well as the trans- 

 ference of the disease so caused from one man to another; 

 but the mites do not propagate on the human skin, and the 

 symptoms of their presence spontaneously disappear about the 

 twelfth day. 



The sarcoptes equi is transmissible to horned cattle ; but it 

 is as yet doubtful if the dog, cat, and pig are susceptible of 

 the mange induced by it. Gerlach failed to communicate the 

 disorder to the sheep, although the mites were by him placed 

 on the skin. 



THE DERMATODECTES EQUI. 



The dermatodectes equi, which has hitherto been looked 

 upon as the veritable sarcoptes or acarus equi, does not live, 



