PART EIGHTH. 

 EXTERNAL INTEGUMENT. 



THE SKIN. 



THE skin forms one of the principal surfaces by means of 

 which the organism comes in contact with the ambient 

 mediums : therefore, we shall proceed to study ; first its struc- 

 ture, and then its functions in regard to the exchanges which 

 take place, either from within to without, or from without to 

 within ; and finally its sensibility, or the power which it has 

 of conveying the impression of the outer world to the origins 

 of the sensory or centripetal nerves. 



I. Structure of the sJcin Epidermic productions. 



a. Dermis and JSpidermis. The skin (Fig. 93) is formed 

 of the dermis and the epidermis. The dermis forms a sub- 

 stratum of connective and elastic tissue, serving as a support 

 for the most important part of the cutaneous covering, the 

 epidermis, and contains blood-vessels, nerves, and the glan- 

 dular organs produced by its deep-seated vegetation. The 

 dermis nlso contains smooth muscular elements, unequally 

 distributed in different parts : in the skin of the scrotum these 

 elements form a continuous layer (dartos). In the nipple, 

 they form a special erectile organ ; above all, they are joined 

 to the follicles of the hair, which they can straighten ; the 

 contraction of these muscles, under the influence of cold, for 

 instance, produces the sensation known as having one's flesh 

 creep, goose-flesh. This sensation, as well as the erection of 

 the nipple, are purely muscular phenomena, and in no way 

 resemble the erection of the erectile vascular tissues: the 

 nipple, for instance, has transverse muscular fibres, the con- 

 traction of which increases its length by diminishing its 

 thickness; in the case of the phenomenon of goose-flesh, 



