SECTION XL 

 THE CUTANEOUS FUNCTIONS. 



ANOTHER source of loss to the blood occurs in its passage through 

 the capillaries of the external integument by means of the sweat and 

 the sebaceous glands. As already indicated, the waste products of the 

 animal economy are mainly urea and its antecedents, carbon dioxide, 

 salts, and water. In the study of respiration we found that the pul- 

 monary mucous membrane constituted the organ whose function as an 

 excretory organ was mainly concerned in the removal of carbon dioxide, 

 water, and certain organic products from the blood. The kidney we 

 found to be especially active in removing urea, various salts, and water. 

 These two organs constitute the main paths of elimination of substances 

 no longer of use to the economy, and, as a consequence, constitute the 

 most important excretory organs of the body. 



The skin may be regarded as an organ supplemental in its action 

 to the lungs and kidneys, since the skin by its secretion is capable of 

 removing a considerable quantity of water from the blood, small amounts 

 of carbon dioxide, and small amounts of salts, and in certain instances 

 during suppression of renal secretion a small amount of urea. 



The skin is thus an excretory organ, serving to remove gaseous, 

 liquid, and solid waste products. The skin is also the chief organ for the 

 regulation of animal heat, by, on the one hand, through conduction, 

 radiation, and evaporation of water, permitting of loss of heat, while it 

 also, through mechanisms to be considered directly, is able to regulate 

 the amount of heat lost. Since the skin is more permeable to gases 

 than a dry membrane, a certain amount of gaseous interchange takes 

 place through the skin. The skin further, through the various forms 

 which the epidermal organs may take on, whether hoofs, horns, claws, 

 fur, or feathers, furnishes means of protection and offensive organs. 

 Thus, the hairs (fur or feathers) furnish protection against extreme and 

 sudden variations of temperature by the fact that they are poor con- 

 ductors of heat, and inclose between them a still layer of air, itself a 

 poor conductor of heat. The hairs are also furnished with an apparatus 

 by which the loss of heat may be regulated ; thus, in cold weather, through 

 the contraction of the unstriped muscular fibres of the skin (the erectores 

 pili}, the hairs become erect and the external coat thus becomes thicker. 

 Further, cold acts as a stimulus to the growth of hair, and we find, as a 



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