608 THE SKIN AND ITS APPENDAGES [CH. XL. 



The sebaceous glands (fig. 389) are small saccular glands, with 

 ducts opening into the upper portion of the hair follicles. The 

 secreting cells become charged with fatty matter, which is discharged 

 into the lumen of the saccules owing to the disintegration of the cells. 

 The secretion, sebum, contains isocholesterin (see p. 436) in addition 

 to fatty matter. It acts as a lubricant to the hairs. 



The sweat-glands (fig. 388) are abundant over the whole human 

 skin, but are most numerous where hairs are absent, on the palms 

 and soles. Each consists of a coiled tube in the deepest part of the 

 dermis, the duct from which passes up through the dermis, and by a 

 corkscrew-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 on Temperature. 



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 T ^o- 

 to 2ihr of that which passes from the lungs. But in thin-skinned 

 animals, such as 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 discharged 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 are 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. 



