THE SKIN. 285 



duct is formed of the cells of tlie corneous layer, and the duct 

 opens on the free surface at the summit of the ridges. 



The formation of the sweat-glands commences in the fifth 

 month of foetal life by the pushing of epithelial cells from the 

 rete mucosum into the cutis. In the seventh month the epi- 

 thelial cells form a canal, and the lower end of the tube be- 

 comes dilated and somewhat twisted. In the ninth month the 

 tube is coiled upon itself to form the gland proper. According 

 to Ranvier, who believes that the muscular fibres lie between 

 the epithelial cells and the basement-membrane, the muscle- 

 cells arise from the external cells of the gland proper by a 

 process of simple differentiation. The lumen of the tube is 

 formed not by a softening down of the central cells, but by the 

 formation of the cuticula, which occurs first at the lowest part 

 of the excretory duct (Ranvier). 



The sebaceous glands. The sebaceous glands are seated in 

 the corium and are in close connection with the hair-follicles. 

 When the hairs are large the sebaceous glands appear as ap- 

 pendages to the hair-follicles into which their ducts enter, and 

 by which their contents are carried to the free surface. As 

 regards the small downy, or lanugo hairs, they may be said to 

 open into the ducts of the sebaceous glands, the ducts of the 

 latter having in this case a much greater diameter than in the 

 previous instance. They also open directly on the free surface. 



The sebaceous glands are almost without exception acinous 

 glands, the number of lobules forming a single gland, ranging 

 from two to twenty, or more. The largest glands are seated in 

 the nose, cheeks, scrotum, about the anus, and in the labia. 

 Occasionally the secreting portion of a sebaceous gland con- 

 sists of a single tubule, or sac, whose duct opens into a hair- 

 follicle. 



Every sebaceous gland is composed of two parts, viz.: the 

 secreting portion, or gland proper, and the duct. The gland 

 proper is formed of a basement-membrane, or sac, externally, 

 and secreting cells, or their products, internally. The basement- 

 membrane is continuous with the transparent membrane de- 

 scribed as lying directly beneath the rete Malpighii and above 

 the corium, and has a similar structure. This basement-mem- 

 brane passes from the sebaceous gland to the hair-follicle, where 

 it forms the inner layer of the hair-sac. The membrane of the 

 sebaceous gland is surrounded externally by bands of dense 



