THE SKIN. 211 



between the cells of the cortex, and are filled with air, reflecting the 

 light and masking the pigmentation of the cells. 



The nails are especially thick and condensed masses of epithelial 

 cells which have undergone keratoid degeneration and are closely 

 compacted. They are produced at the root of the nail, and as they 

 accumulate push the body of the nail forward. They, therefore, 

 correspond to the horny layer of the epidermis, which has become 

 modified to form these special structures (Fig. 188). 



The skin contains little muscular bands, the arrectores pili (Fig. 

 181, mh), composed of smooth muscular fibres, which are attached 

 to the fibrous coat of the hair- follicles near their deep extremities 

 and to the superficial layer of the corium on the side of the fol- 

 licle toward which the hair leans. The action of these mus- 

 cles is to cause the hair to assume a more vertical position, and 

 to raise it and the follicle, producing the effect known as " goose 

 flesh." By their contraction they may also aid in the discharge 

 of sebum, since their fibres often partially invest the sebaceous 

 glands. 



The functions of the skin have reference to its being the organ 

 coming in contact with the external world. The epidermis protects 

 the underlying tissues from mechanical and chemical injury and 

 from desiccation. The keratin in its horny layer forms an imper- 

 vious and tough investment of the body, which is highly resistant 

 toward chemical action and mechanical abrasion, and is constantly 

 renewed from the layers that lie beneath it. It is kept in a pliable 

 condition by the sebum discharged upon its surface and by the 

 moisture proceeding from the sweat-glands, the " insensible perspi- 

 ration." The skin also plays a prominent role in the regulation of 

 the bodily temperature. When its vessels are contracted the amount 

 of heat given off from the surface of the body is reduced ; when 

 they are dilated, it is increased. A further loss of heat is occa- 

 sioned by an increased secretion of sweat, which bathes the surface 

 of the skin and abstracts from the body the heat required to con- 

 vert it into vapor. Under the influence* of sudden and marked 

 cold the vessels of the skin become much contracted and the 

 arrectores pili shorten, occasioning the production of a roughness 

 of the skin, goose-flesh, and probably also a discharge of se- 

 bum, which reduce the evaporation from the skin. At the same 

 time a reflex rhythmical contraction and relaxation of the volun- 

 tary muscles is brought about — shivering, which increases the 



