128 OF THE SECRETIONS. 



living tissues. The bloodvessels, especially the capillaries, 

 share this property. Hence portions of the circulating fluids 

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

 surface. This superficial loss is termed exhalation. It is 

 most active where the bloodvessels most abound, and accord 

 ingly is very copious from the air-tubes of the lungs and 

 from the skin. The loss in this way is very considerable ; 

 and it has been estimated that, under certain circumstances, 

 the body loses, by exhalation, five eighths of the whole weight 

 of the substances received into it. 



267. The skin, or outer envelop of the body, is otherwise 

 largely concerned in the losses of the body. Its layers 

 are constantly renewed by the tissues beneath, and the 

 outer dead layers are thrown off. This removal is some- 

 times gradual and continual, as in man. In fishes and many 

 mollusks, it comes off in the form of slime, which is, in fact, 

 composed of cells detached from the surface of the skin. 

 Sometimes the loss is periodical, when it is termed moulting. 

 Thus, the mammals cast their hair, and the deer their horns, 

 the birds their feathers, the serpents their skins, the crabs 

 their test, the caterpillars their outer envelop, with all the 

 hairs growing from it. 



268. The skin presents such a variety of structure in the 

 different groups of animals as to furnish excellent distinctive 

 characters of species, genera, and even families, as will 

 hereafter be shown. In the vertebrates we may recognize 

 several distinct layers, of unequal thickness, as may be seen 

 in figure 94, which represents a magnified section of the 

 human skin, traversed by the sudoriferous canals. The 

 lower and thickest layer, (a,) is the cutis^ or true skin, and 

 is the part which is tanned into leather. Its surface presents 

 numerous papillae, in which the nerves of general sensation 

 terminate ; they also contain a fine network of bloodvessels, 



