DISEASES OF THE SKIN. 321 



to the root bulb, and the ducts of one or more sebaceous glands open 

 into it. At the bottom of each follicle is a concial, vascular papilla, 

 similar in every respect to those on the surface of the dermis; this 

 papilla fits into a corresponding depression in the root of the hair. 

 The shaft consists of a center, or medulla, a surrounding fibrous 

 portion, and an external coating, or cortex. The medulla consists 

 of cells containing pigment or fat, is opaque, and deeply colored. 

 All kinds of hair do not have this medulla. The fibrous portion 

 occupies the bulk of the stem, and the cortex is merely a single 

 laj-er of thin, flat, imbricated (shinglelike) scales. 



The sebaceous glands, lodged in the corium, are most abundant in 

 parts exposed to friction. They generally open into the hair follicles, 

 occasionally on the surface of the body. Each gland consists of a 

 small duct which terminates in a lobulated recess. These lobules 

 vary, and are, as is the duct, lined with epithelium. They are filled 

 with sebaceous matter which, as it is secreted, is detached into the 

 sacs. They are very plentiful between the claws of cattle. 



The sudorific glands, or sweat glands, are situated in the subcuta- 

 neous areolar tissue, surrounded by a quantity of fat. They are 

 small, round, reddish bodies, each of which consists of one or more 

 fine tubes coiled into a ball, the free end of the tube being continued 

 up through the true skin and cuticle, and opening on the surface. 

 Each sweat gland is supplied with a cluster of capillary blood ves- 

 sels which vary in size, being very large when perspiration is exces- 

 sive. The contents of the smaller ones are fluid, and of the larger, 

 semifluid. 



The skin may be regarded as an organ supplementary in its action 

 to the kings and kidneys, since by its secretion it is capable of remov- 

 ing a considerable quantity of water from the blood; it also removes 

 small quantities of carbon dioxid, salts, and in certain instances 

 during suppression of the renal secretions a small quantity of urea. 

 The skin is also the chief organ for the regulation of animal heat, 

 by or through conduction, radiation, and evaporation of water, per- 

 mitting of loss of heat, while it also, through other mechanisms, is 

 able to regulate the heat lost. The hair furnishes protection against 

 extreme and sudden variations of temperature by reason of the fact 

 that hairs are poor conductors of heat, and inclose between them a 

 still layer of air, itself a nonconductor. The hairs are also furnished 

 with an apparatus by Avhich the loss of heat may be regulated; thus, 

 in cold weather, through the contraction of unstriped muscular fibers 

 of the skin, the hairs become erect and the external coat becomes 

 thicker. Cold, too, acts as a stimulus to the gi^owth of hair, and we 

 find, in consequence, a thicker coat in winter than in summer. The 

 hairs also furnish protection against wet, as they are always more 

 or less oily from the secretion of sebaceous glands, and thus shed 



54793"— 23 21 



