1394 



HUMAN ANATOMY. 



the cells overlying the summit of the papilla, while those converted into the cortical substance, 

 cuticle and inner root-sheath occupy the sides of the papilla and deepest part of the follicle. 



With few exceptions, the hair foUicles are associated with two or more 

 sebaceous glands, rarely with only one, the ducts of which open into the 

 sac in the vicinity of the neck. The glands usually lie on the side towards which 

 the hair inclines, but sometimes, especially in the case of the smaller hairs, they may 

 completely surround the follicle. Since these glands are outgrowths from the same 

 tissue that lines the follicles, their ducts pierce the outer root-sheath, bringing their 

 oily secretion into direct relation with the hairs. 



The structure of the sebaceous glands is described with the cutaneous glands 

 (page 1397). 



Most of the larger hair-follicles, particularly those of the scalp, are provided with 

 ribbon-like bundles of involuntary muscle, called the arrectores pilorum in recog- 

 nition of their effect on the hairs. They arise from the superficial part of the corium, 



passobliquely downward to be inserted 

 Fig. 1 158. jji^Q (.j^g sheath of the hair-follicle near 



the junction of corium and subdermal 

 tissue and on the side corresponding 

 with the inclination of the hair and 

 the situation of the sebaceous glands. 

 Since the latter are closely embraced 

 by the muscular bands, contraction oi 

 the muscles exerts pressure upon the 

 glands and facilitates the discharge of 

 their secretion {sebum) — hence these 

 muscles are sometimes also designated 

 expressores sebi. The effect of con- 

 traction of the arrectores pilorum is 

 oftenconspicuouslyseen on the surface 

 in the condition known as "goose- 

 flesh' ' {cutis anserina) ,v;'\\^x(t\.\\^ hairs 

 and surrounding tissue appear to be 

 unusually elevated owing to the 

 upward pull on the hair-follicles and 

 the consequent erection of the hairs 

 in the opposite direction. 



The blood-vessels supplying 

 the hair-follicle, which in a sense con- 

 stitute a special system for each sac, include the capillary loops ascending within the 

 hair-papilla and the net-work of capillaries surrounding the follicle immediately outside 

 the glassy membrane. The first are derived from a small special twig that ascends 

 to the follicle, and the second from the subpapillary net-work of the corium. With 

 the exception of those draining the papilla, which are tributary to the deeper stems, 

 the veins join the subpapillary plexus. 



The nerves distributed to the follicles follow a fairly definite arrangement. As 

 shown by Retzius, usually each hair-sac is supplied by a single fibre, sometimes by 

 two or more, which approaches the follicle immediately below the level of the mouth 

 of the sebaceous glands. After penetrating the fibrous sheath as far as the glassy 

 membrane, the nerve-fibre separates into two divisions that encircle more or less 

 completely the follicle and on the opposite side break up into numerous fibrillae 

 constituting a terminal arborization. The nerve-endings usually lie on the outer 

 surface of the glassy membrane within the middle third of the follicle and only 

 exceptionally are found within the outer root-sheath or the hair-papilla. 



THE NAILS. 



The nails (ungues), the horny plates overlying the ends of the dorsal surfaces of 

 the fingers and toes, correspond to the claws and hoofs of other animals and, like 

 them, are composed exclusively of epithelial tissue. They are specializations of the 



Portion of section of injected scalp, showing capillary 

 net-works surrounding hair-follicles and twigs entering 

 papillae. X 20. 



