57 6 THE SKIN. [CH. xxxvni. 



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 ; 



Kg. 446. Terminal tubules of sudoriferous or sweat-glands, cut in various directions 

 from the skin of the pig's ear. (V. D. Harris.) 



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 Begulation. 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 -j-4-o to 2^0 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 (Bischoft). 



Absorption. This also is an unimportant function ; but the 

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



