44 JOURNAL OF THE [April, 



off. Nerves, glands, pigment cells, bony structures, and blood 

 vessels occur principally in the dermis. So-called epidermic 

 structures, such as skin glands, hairs, feathers, nails, hoofs, 

 claws, bristles, etc., are formed from the epidermis or outer layer. 

 These will be treated of in those types in which they are most 

 •characteristic, and we can then also see how environment, both 

 natural and artificial, has brought about many changes both in 

 their form and function. This need not appear strange or won- 

 derful when we consider how accessible the outer surface of the 

 skin is to external modifying influences. Before beginning with 

 our types of animal life and illustrating the modifications that 

 occur, the two primary divisions of the skin must be borne in 

 mind — an outer layer, protective, and an inner layer, nutritive 

 in function. 



A few words in regard to the general physiology. Broadly 

 speaking, the waste products of the body are urea, carbon di- 

 oxide, water, and various salts. These leave the body by one or 

 other of three main channels : the luugSy the kidneys, or the skin. 

 The lungs discharge most of the carbon dioxide and some water ; 

 the kidneys, the urea and allied bodies ; the skin, a small amount 

 of the salts and nearly all the jvater. The skin is therefore the 

 great evaporating agent, and the discharge of waste products by 

 this channel we know as perspiration or sweat. 



It has been proven that death would ensue in an animal in 

 which this cutaneous evaporation was prevented by covering its 

 body with an impermeable varnish which retained the sweat in 

 the glands, which thus acted as a poison. 



The skin in the lower forms can also take the place of lungs. 

 If the lungs of a frog be removed he will continue to live for 

 some time, consume oxygen, and produce carbon dioxide, as in 

 the ordinary mode of breathing, thus showing that respiration 

 can be carried on efficiently by means of the skin. 



Having briefly noticed its main anatomical and physiological 

 features, we will now take up the skin comparatively in the five 

 great classes of the vertebrates, and examine its most prominent 

 characters in each. 



