380 EXCRETION. 



Under the head of excretion, it is proposed to consider the general properties and 

 composition of the different excrementitious fluids ; but the relations of the excremen- 

 titious matters themselves to the tissues will be more fully treated of in connection with 

 nutrition. 



The urine is a purely excrementitious fluid. The perspiration and the secretion of the 

 axillary glands are excrementitious fluids, but they contain a certain amount of the secre- 

 tion of the sebaceous glands. Certain excrementitious matters are found in the bile, but, 

 at the same time, this fluid contains principles manufactured in the liver and has an impor- 

 tant function as a secretion, in connection with the process of digestion. 



Physiological Anatomy of the Skin. 



The skin is one of the most complex and important structures in the body, and it pos- 

 sesses a variety of functions. In the first place, it forms a protective 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 tactile sensibility, the skin has an important 

 function, being abundantly supplied with sensitive nerves, some of which present an 

 arrangement peculiarly adapted to the nice appreciation of external 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 very closely attached to the subjacent struct- 

 ures, 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 and probably not subject to 

 much variation, the evaporation of water from the general surface is always considerable 

 and is subject to such modifications as may become necessary from the varied conditions 

 of the animal temperature. Thus, while the skin protects the body from external influ- 

 ences, its function is important in regulating the heat produced as one of the numerous 

 phenomena attendant upon the general process of nutrition. 



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

 conveniently considered in connection with different divisions of the subject of physi- 

 ology. For example, under the head of secretion, we have already taken up the struct- 

 ure of the different varieties of sebaceous glands ; and the anatomy of the skin as an organ 

 of touch will be most appropriately considered in connection with the nervous system. 

 In this connection, we shall describe the excreting organs found in the skin ; and here it 

 will be most convenient to study 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 very careful 

 observations upon the extent of the surface of the skin. Without detailing the measure- 

 ments 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 little more than sixteen square feet ; 

 and, in men of more than ordinary size, it may extend to twenty-one or twenty-two 

 square feet. In women of medium size, as the mean result of three observations, the 

 surface was found to equal about twelve square feet. When we consider the great extent 

 of the cutaneous surface, it is not surprising that the amount of secretion, under certain 

 conditions, should be enormous. Indeed, under all circumstances, the amount of elimina- 

 tion is very considerable, and the skin is really one of the most important of the organs 

 of excretion. 



