CHAPTER XII 

 EXCRETION BY THE SKIN 



Physiological anatomy of the skin Layers of the skin The corium, or true skin The epi- 

 dermis Physiological anatomy of the nails Physiological anatomy of the hairs Roots 

 of the hairs, and hair-follicles Growth of the hairs Sudden blanching of the hair 

 Uses of the hairs Perspiration Quantity of cutaneous exhalation Properties and 

 composition of the sweat. 



PHYSIOLOGICAL ANATOMY OF THE SKIN 



THE skin is one of the most complex and important structures in the 

 body and has a variety of uses. In the first place, it forms a protec- 

 tive covering for subjacent parts. It is quite thick over the parts most 

 subject to pressure and friction, is elastic over movable parts and those 

 liable to variations in size, and in many situations is covered with hair. 

 The skin and its appendages are imperfect conductors of caloric, are 

 capable of resisting very considerable variations in temperature, and 

 they thus tend to maintain the normal standard of the animal heat. As 

 an organ of sensibility, the skin has important uses, being abundantly 

 supplied with sensory nerves, some of which present an arrangement 

 peculiarly adapted to the nice appreciation of tactile impressions. The 

 skin assists in preserving the external forms of the muscles. It also 

 relieves the abrupt projections and depressions of the general surface 

 and gives roundness and grace to the contours of the body. In some 

 parts it is closely attached to the subjacent structures, while in others 

 it is less adherent and rests on a layer of adipose tissue. 



The skin is important as an organ of excretion ; and although the 

 quantity of excrementitious matter exhaled from it is not large, the 

 evaporation of water from the general surface is considerable and is sub- 

 ject to such modifications as may become necessary from the varied 

 conditions of the animal temperature. Thus, while the skin protects 

 the body from external influences, its office is important in regulating 

 the heat that is produced as one of the phenomena attendant on the 

 general process of nutrition. 



Extent and Thickness of tJic Skin. Without detailing the measure- 

 ments of different parts, it may be stated in general terms that the 

 cutaneous surface in a good-sized man is equal to a little more than 

 sixteen square feet (15,000 square centimeters); and in men of more 

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