202 THE HUMAN SPECIES 



been keenly debated : experiments show beyond doubt that the 

 skin of animals and man alike allows the passage of gases 

 through its pores. 



After carefully sealing up all mucous apertures it has been 

 found that certain fluids (water and watery solutions) are 

 scarcely or not at all absorbed by the intact skin on account of 

 the natural fat on its surface, while other fluids readily miscible 

 with the fat (alcohol, ether, chloroform, oil of turpentine) are 

 easily absorbed, just as happens with ointments rubbed into the 

 pores of the skin. 



The transpiration of the skin whereby CO 2 and H. 2 O are 

 excreted has been exactly determined by careful experiment 

 in man and animals. The maximum is reached in those lower 

 classes whose integument is a single layer of epithelium ; it 

 becomes less as the epidermis is thicker or a hairy coat is de- 

 veloped. The greater the depth of skin or hair that covers the 

 cutaneous capillaries the less is the amount of transpired CCX. 



In horses, for instance, it is only T i^ of the respirator}' CCX 

 output, while in man in twenty-four hours the amount excreted 

 through the skin is from 5-10 gm., i.e., | to I per cent, of the 

 total CO 2 output. 



The water-loss through the skin is, according to Henneberg : 



In oxen by respiration and perspiration 5-6 kilos daily 



horse (at rest) ,, ,, ,, up to 10 ,, ,, 



(at work) ,, . twice this amount ,, 



man (at rest) ,, . ,, ,, ,, about 600-660 gm. 



These quantities are nearly twice the " water-loss " by the lungs. 1 

 In the sections on Anatomy it has been pointed out that 

 man only possesses a very limited power of moving the skin by 

 subcutaneous muscles. The frog possesses a number of purely 

 cutaneous muscles, many reptiles can move their shells, scales 

 or skin appendages, while birds are provided with many 

 cutaneous muscles for moving their feathers or spreading their 

 wings. In many animals unstriped muscles exist in the skin of 

 the head, neck, shoulders and abdomen, so that in some in- 

 stances (the hedgehog and porcupine) a complete cuirass of 

 cutaneous muscle enables the body to be rolled up in the well- 

 known fashion. 



1 M. Sussdorf in Ellenberger's Physiologic, ii., p. 678. 



