70 ANIMAL PHYSIOLOGY. 



approach the surface, in process of conversion into the horny- 

 layer, become flattened ; the horny wall of the duct, as it is 

 carried to the surface with the structures in which it is im- 

 bedded, having therefore to be accommodated in a shorter 

 vertical depth than it occupied at first, is pressed by the 

 shrinking structures round it into an oblique position, and 

 the regular continuance of this process gives rise to a spiral. 



The sebaceous glands (fig, 41) are for the most part con- 

 nected with hairs ; and large hairs have usually several of 

 them opening into the necks of their follicles, but fre- 

 quently it happens, particularly in the face, that a very 

 large sebaceous gland opens into the neck of the follicle of 

 a very small hair, showing that these glands not only serve 

 to lubricate the hair, but the integument as well. The 

 sebaceous glands consist of one or more small groups of 

 saccules opening into a common duct, lined with epithelium, 

 and filled with oily matter, 



45. The perspiration is the combined product of both sets 

 of glands, but principally is derived from the sudoriparous. 

 The sebaceous glands secrete nothing but oil, and they are 

 not the exclusive source of the oil of the skin ; for not only 

 may particles of oil be detected in the interior of large sweat 

 glands, but oil is secreted by the palms of the hands, which 

 have no sebaceous glands. In connection with this it may 

 be mentioned that the ceruminous glands, which secrete the 

 wax of the ears, are simply large sweat glands. The most 

 abundant solid constituent of the perspiration is common, 

 salt, chloride of sodium; and besides other salts, there is 

 always a small amount of urea in it. Carbonic acid is 

 likewise given off by the skin, although the amount of it 

 is insignificant compared with what escapes by the lungs. 

 The total amount of perspiration is obviously exceedingly 

 variable; but in experiments made by enveloping the body 

 in a water-tight bag with apertures for breathing, -it has been 

 found to be little short of two pounds per diem. One obvious 

 use of the perspiration, and probably the principal purpose 

 which it serves, is the protection of the body from too great 

 heat, whether of external or internal origin, by the cooling 

 effect of its evaporation from the surface, as will be further 

 referred to (p. 149). Its flow, like that of saliva, is probably 



