THE SKIN AND ITS APPKXDAGES 399 



the various parts of the hair and its follicle are direct derivatives of the different 

 layers of the skin, and their growth takes place by a continuous process of invag- 

 ination. Thus the most peripheral cells of the outer root-sheath — stratum 

 cylindricum — pass over the papilla and turn upward to form the medulla of 

 the hair; the deeper cells — stratum spinosum — of the outer root-sheath become 

 continuous with the cortex of the hair; the stratum lucidum, with the sheath of 

 Henle, which turns up on the hair as its cuticle; Huxley's layer, with the cuticle 

 of the inner root-sheath. According to this view growth of hair is accomplished 

 by continuous growth downward from the surface, and turning up into the 

 hair, of these layers. 



TECHNIC 



Pin out small pieces of human scalp on cork and fix in absolute alcohol or 

 in formaHn-]\I tiller's fluid (technic 6, p. 7). From one block cut sections perpen- 

 dicular to the surface of the scalp and in the long axes of the hair and follicles. 

 From a second block cut sections at right angles to the hair follicles, i.e., not quite 

 parallel to the surface of the scalp but a little obliquely. By this means not 

 only are transverse sections secured, but if the block be sufficiently long the fol- 

 licles are cut through at all levels. Sections are stained with haematoxylin- 

 picro-acid-fuchsin (technic 3, p. 21) and mounted in balsam. 



Blood-vessels of the skin. From the larger arteries in the subcu- 

 taneous tissue branches penetrate the pars reticularis of the derma, 

 where they anastomose to form cutaneous networks. The latter give 

 off branches, which pass to the papillary layer of the derma and there 

 form a second series of networks, the subpapillary, just beneath the 

 papilla?. From the cutaneous networks arise two sets of capillaries, 

 one supplying the fat lobules, the other supplying the region of the 

 sweat glands. From the subpapillary netw^orks are given off small 

 arteries which break up into capillary networks for the supply of the 

 papillae, sebaceous glands, and hair follicles. The return blood from 

 these capillaries first enters a horizontal plexus of veins just under 

 the papillae. This communicates with a second plexus just beneath 

 the first. Small veins from this second plexus pass alongside the 

 arteries of the deeper part of the corium, where they form a third 

 plexus with larger, more irregular meshes. Into this plexus pass 

 most of the veins from the fat lobules and sweat glands, although one 

 or two small veins from the sweat glands usually follow the duct and 

 empty into the subpapillary plexus. The blood next passes into a 

 fourth plexus in the subcutaneous tissue, from which arise veins of 

 considerable size. These accompany the arteries. 



Small arteries from the plexuses of the skin and subcutis pass to 

 the hair follicle. The larger arterioles run longitudinally in the outer 



