CH. XXXVIII.] FUNCTIONS OF THE SKIN 579 



duct from which passes up through the dermis, and by a cork sere w- 

 like canal through the epidermis to the surface. 



The secreting tube is lined by one or two layers of cubical or 

 columnar cells; outside this is a layer of longitudinally arranged 

 muscular fibres, and then a basement-membrane. 



The duct is of similar structure, except that there is usually but 

 one layer of cubical cells, and muscular fibres are absent ; the passage 

 through the epidermis has no proper wall ; it is merely a channel 

 excavated between the epidermal cells. 



The ceruminous glands of the ear are modified sweat-glands. 



THE FUNCTIONS OF THE SKIN 



Protection. The skin acts as a protective organ, not only by 

 mechanically covering and so defending internal structures from 

 external violence, but more particularly in virtue of its being an organ 

 of sensation (see later in the chapter on Touch). 



Heat Regulation. See Chapter XL. 



Respiration. A small amount of respiratory interchange of gases 

 occurs through the skin, but in thick-skinned animals this is very 

 small. In man, the carbonic acid exhaled by the skin is about y-l^ 

 to oiro f that which passes from the lungs. But in thin-skinned 

 animals, like frogs, cutaneous respiration is very important ; after the 

 removal of the lungs of a frog, the respiratory interchange through 

 the skin is sufficient to keep the animal alive, the amount of carbonic 

 acid formed being about half as much as when the lungs are present 

 (Bischoff). 



Absorption. This also is an unimportant function ; but the skin 

 will in a small measure absorb oily materials placed in contact with 

 it ; thus in some cases infants who will not take cod-liver oil by the 

 mouth, can yet be dosed with it by rubbing it into the skin. Many 

 ointments also are absorbed, and thus general effects produced by 

 local inunction. 



Secretion. The secretions of the skin are two in number. The 

 sebum is the natural lubricant of the hairs. The secretion of sweat is 

 an important function of the skin, and we will therefore discuss it at 

 greater length. 



THE SWEAT 



Physiology of the Secretion of Sweat. We have seen that the 

 sweat-glands are most abundant in man on the palms and soles, and 

 here the greatest amount of perspiration occurs. Different animals 

 vary a good deal in the amount of sweat they secrete, and in the 

 place where the secretion is most abundant. Thus the ox perspires 

 less than the horse and sheep; perspiration is absent from rats, 



