Human physiology : designed for colleges and the higher classes in schools and for general reading . spiration. The perspira-tion is ordinarily insensible, as it is termed ; that is, it is in theform of vapor. But sometimes, as in vigorous exercise, whenthe sweat glands are rendered very active, chiefly to prevent toogreat an accumulation of heat, the perspiration becomes There is another set of glands in the skin calledsebaceous glands, which secrete an oily fluid. They havealso thin tubes like the sweat glands. They are mostabundant where the skin specially needs an oily lubric
Human physiology : designed for colleges and the higher classes in schools and for general reading . spiration. The perspira-tion is ordinarily insensible, as it is termed ; that is, it is in theform of vapor. But sometimes, as in vigorous exercise, whenthe sweat glands are rendered very active, chiefly to prevent toogreat an accumulation of heat, the perspiration becomes There is another set of glands in the skin calledsebaceous glands, which secrete an oily fluid. They havealso thin tubes like the sweat glands. They are mostabundant where the skin specially needs an oily lubrica-tion, as where there are folds in the skin or hairs, orwhere the skin is exposed to friction, or to the drying atmos-phere. They are very abundant on the face and head. Theamount of the oily secretion of these glands is very great inthe skin of races fitted to inhabit warm climates. Every hairhas sebaceous glands connected with it, as represented inFig. 59 ; in which b is the hair emerging from the skin; FORMATION AND REPAIR. 121 Influence of labor on weur and tear, and on absorption. FIG. C HAIR and sebaceous glands. a a are the sebaceous glands pouring theirsecretion by thin tubes into the tube orcanal in which the hair grows ; c the rootof the hair surrounded with fat all this you see that the skin, withits two sets of glands and tubes, its nerv-ous papillae, and its layers of constantlyrenewed cells, making the cuticle, is acomplicated organ, and is thus fitted toperform its functions as an organ of sen-sation, and at the same time of excretion,while it is also a pliable but firm cover-ing for the body. 182. You have seen in the facts de-veloped in this chapter, that there is con-stant change going on in all parts of thebody. Particles which have become use-less are taken up by the absorbents, whilethe formative vessels deposit others to taketheir places. The rapidity with whichthis change occurs, depends mostly uponthe activity of the individual. The b
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