EXCRETION 405 



The Liver and Intestines. The liver and intestines are both 

 concerned with the removal of bile, a part of which is waste matter, 

 and the intestines also remove the unused food refuse, which, how- 

 ever is not strictly excretion. 



The Skin. The skin excretes considerable water and only 1 per 

 cent of solid matter, mainly salts, and a very little urea. The 

 chief function of perspiration is to regulate the temperature of the 

 body. 



Structure. While not primarily an organ of excretion, the struc- . 

 ture and functions of the skin may be discussed at this point. 

 The human skin is a much thicker and more important organ than 

 we usually suppose. When tanned into leather it resembles the 

 pig-skin cover of a foot ball. 



It consists of an outer portion (epidermis) composed of many 

 layers of cells, the outer-most, dead, horny scales, the inner ones, 

 more active and larger. Its function is mainly protective and the 

 outer scales are constantly being rubbed off and replaced by new 

 from beneath. Where subject to much friction or pressure the epi- 

 dermis may grow to over a hundred cell layers in thickness, pro- 

 ducing the familiar callouses of hands and feet. 



Hair, nails, and color cells are developed from the epidermal 

 layer in man. Scales, feathers, and claws are modified forms found 

 in other animals. 



Beneath the epidermis is a thicker layer (the dermis) consist- 

 ing of tough fibrous connective tissue, richly supplied with blood 

 and lymph vessels, nerves, sweat, and oil glands. 



Functions of the Skin. These include: 



1. Protection from germ attack and mechanical injury. 



2. Protection of inner tissues from drying. The skin, aided by 

 the oil glands, is nearly water proof, neither absorbing nor letting 

 out moisture, except at the sweat pores. 



3. It is the location of most of our nerves of touch. 



4. Excretion of sweat as a waste matter. 



5. Excretion of sweat to regulate the temperature of the body. 

 This last statement needs explanation. Birds and mammals 



are the only animals whose temperature does not change with 



