DISEASES OF THE SKIN. 821 



sists of a center, or mechilla, 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 layer of thin, flat, imbricated 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, red'dish 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 vessels which vary 

 in size, being Very large when perspiration is excessive. 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 lungs and kidneys, since the skin by its secretion is capable 

 of removing a considerable quantity of water from the blood, small 

 amounts of carbon dioxide, and small amounts of salts, and in certain 

 instances during suppression of the renal secretions, a small amount 

 of urea. The skin is also the chief organ for the regulation of animal 

 heat, by or through conduction, radiation, and evaporation of water, 

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

 able to regulate the amount of heat lost. The hair furnishes protec- 

 tion against extreme and sudden variations of temperature by the 

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

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

 furnished with an apparatus by which the loss of heat may be regu- 

 lated; 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 growth 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 water. The hairs, through their elasticity, furnish mechani- 

 cal protection, and through the thickness of the coat, to a certain 

 degree, resist the attacks of insects. Finally, the hairs assist the 

 sense of touch. 



The sweat glands are constantly discharging a watery secretion in 

 the form of insensible perspiration, and by their influence act as 

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