OF THE SECRETIONS. 99 



may be seen by Fig. 94, which represents a magnified sec- 

 tion of the human skin traversed by the sudoriferous canals. 

 The lower layer or the leather (a) is the thickest ; it covers 

 the muscles, from which it is separated by a bed of fat 

 in which the glands of transpiration are situated. Above the 

 leather is a thinner layer, the vascular layer (Z), so called 

 from the abundance of blood vessels it contains ; it is also 

 traversed by numerous nerves, which render it very sensi- 

 tive. The third or superficial layer is called epidermis (c). 

 It contains neither nerves nor blood vessels, and conse- 

 quently is insensible. The scales of reptiles, the nails of 

 mammals, and the solid envelops of the Crustacea are merely 

 indurated products of the epidermis. On the other hand, 

 the feathers of birds and the scales of fishes belong to the 

 vascular layer. 



/ 



264. Besides these general functions of the skin, nature 

 has provided several other means for carrying out of the 

 system the superfluous parts, the most important of which 

 are exhalation and secretion. We have already seen (37) 

 that there is a general property of all animal tissues, called 

 endosmosis and exosmosis, by which they may be traversed 

 by liquids and gases. The blood vessels, especially the 

 capillary vessels, share this property of permeability to 

 liquids. Hence, while the circulation goes on, portions of 

 the circulating fluid, especially its watery parts, escape 

 through the walls of the vessels, and pass off at the 

 surface. This superficial loss, termed exhalation, is most 

 active where vessels most abound, and accordingly most 

 copious from the surface of the lungs. It has been esti- 

 mated that, under certain circumstances, the human body 

 loses, by exhalation, five eighths of the whole weight 

 of substances taken into it. 



265. SECRETION is a more complicated process than ex- 

 halation. It is not a mere mechanical operation, but is ac- 



