34:2 EXCEETION BY THE SKIN AND KIDNEYS. 



secretion of the axillary glands are excrementitious fluids, but they contain a 

 certain quantity of the secretion of the sebaceous glands. Certain excre- 

 mentitious matters are found in the bile, but at the same time, this fluid con- 

 tains substances that are formed in the liver, and it has an important office 

 as a secretion, in connection with the processes of digestion. 



PHYSIOLOGICAL ANATOMY OF THE SKIN, 



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

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

 ive covering for the general surface. 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, which 

 affords an additional protection to the subjacent structures. 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 gen- 

 eral surface and gives roundness and grace to the contours of the body. In 

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

 others it is less adherent and is provided with a layer of adipose tissue. 



As an organ of excretion, the skin is very important ; and although the 

 quantity of excrementitious matter exhaled from it is not very great, the 

 evaporation of water from the general surface is always considerable and is 

 subject to such modifications as may become necessary from the varied con- 

 ditions of the animal temperature. Thus, while the skin protects the body 

 from external influences, its office is important in regulating the heat pro- 

 duced as one of the phenomena attendant upon the general process of nu- 

 trition. 



As the skin presents such a variety of uses, its physiological anatomy is 

 most conveniently considered in connection with different divisions of the 

 subject of physiology. For example, under the head of secretion, the struct- 

 ure of the different varieties of sebaceous glands has already been described ; 

 and the anatomy of the skin as an organ of touch will be most appropriately 

 considered in connection with the physiology of the nervous system. In 

 connection with the excreting organs found in the skin, it will be convenient 

 to describe briefly its general structure and the most important points in the 

 anatomy of the epidermic appendages. A full and connected description of 

 the skin and its appendages belongs properly to works upon anatomy. 



Extent and Thickness of the Skin. Sappey has made a number of obser- 

 vations upon the extent of the surface of the skin. Without detailing the 

 measurements of different parts, it may be stated, as the general result of his 

 observations, that the cutaneous surface in a good-sized man is equal to a lit- 

 tle more than sixteen square feet (15,000 square centimetres) ; and in men of 



