CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 691 



basement membrane and the epithelial colls, or rather imbed- 

 ded in the basement membrane, are seen a number of plain 

 muscular fibres disposed longitudinally or in an elongated spiral, 

 and often forming a distinct coat beneath the epithelium. 



As in the case of other glands, we are unable to make any 

 statement as to the work carried on by the epithelium lining the 

 duct, but we may probably assume that the sweat is mainly 

 secreted by the larger cells of the terminal coiled part of the 

 tubule. These cells therefore like other secreting cells are pro- 

 bably ' loaded' and ' discharged' ; but as yet no marked structural 

 changes in the cells corresponding to these phases have been 

 satisfactorily ascertained, though after the administration of 

 pilocarpine, which causes sweating, the cells of glands hardened 

 in alcohol stain more deeply than usual with carmine. It must 

 be remembered, however, that the sweat contains normally neither 

 mucus nor proteid substances, and we should therefore not expect 

 to observe ' granules ' in the cells. 



The peculiarly placed muscular fibres have been supposed, 

 by their contraction, to assist in the flow of sweat along the 

 tubule. In certain cutaneous glands of the frog, of a relatively 

 simple nature, there is evidence that the secretion is ejected from 

 the comparatively large lumen by the contraction of plain muscular 

 fibres in the wall of the gland, or by a contraction of the wall itself, 

 which is contractile without being distinctly differentiated into 

 muscular tissue. And this rather supports the above view ; but 

 the matter is at present by no means clear. 



The coil of a sweat-gland is well supplied with blood vessels 

 in the form of capillary networks, and nerves have been traced to 

 the tubes ; but the exact manner in which these end is not as yet 

 known. 



Though present in all regions of the skin (of man), the sweat- 

 glands are unequally distributed, being more abundant in some 

 regions, such as the palms of the hand, than in others. In the 

 axilla are glands of very large size, and in these the ducts possess 

 distinctly muscular coats. 



437. Sebaceous glands. These are appendages of the hairs. 

 A hair is a development, in the form of a cylinder, of a cap 

 of corneous epidermis surmounting a papilla of the dermis sunk to 

 the bottom of a tubular pit, or involution of the skin, called a 

 hair follicle. In the upper part of the hair follicle the walls 

 consist of ordinary skin with all its parts, dermis, Malpighian laver 

 and corneous layer, the latter as usual of considerable thickness. 

 At some little distance from the mouth of the follicle the corneous 

 layer suddenly ceases, and in the follicle below this the epidermis 

 is represented by the Malpighian layer, now called the outer root- 

 sheath, and two layers of peculiar cells, forming the inner root- 

 sheath, of which the outer is called Henle's and the inner Huxley's 

 layer; these may perhaps be considered as corresponding to the 



442 



