THE HAIRS. 1391 



means always, in harmony with the degree of general pigmentation. The latter is 

 commonly uniform throughout the length of the hair, but in rare cases it may be so 

 variable that the shaft presents a succession of alternating light and dark zones 

 (Brunn). The straight and curly varieties of hair depend chiefly upon differences in 

 the curvature of the follicle l and the form of the hair. In the case of straight hairs 

 the follicle is unbent and the shaft is cylindrical, and therefore circular in cross- 

 section ; hairs that are wavy or curly spring from follicles more or less bent and are 

 flattened or grooved, with corresponding oval, reniform, irregularly triangular or 

 indented outlines when transversely cut. 



Arrangement of the Hairs. Since the buried part of the hair, the root, is 

 never vertical but always oblique to the surface of the skin, it follows that the free 

 part, the shaft, is also inclined. The direction in which the hairs point, however, is 

 by no means the same all over the body, but varies in different regions although 

 constant for any given area. This disposition depends upon the peculiar placing 

 of the hair-roots which in certain localities incline towards one another along 

 definite lines, an arrangement that results in setting the shafts in opposite directions. 

 As these root-lines are not straight but spiral, on emerging from the skin the hairs 

 diverge in whorls (vortices pilorum), the position and number of which are fairly 

 definite. 



Such centres include: (i) the conspicuous vertex whorl on the head, usually single but 

 sometimes double; (2) the facial whorls surrounding the openings of the eyelids; (3) the 

 auricular whorls at the external auditory meatus ; (4) the axillary whorls in the armpits ; and 

 (5) the inguinal whorls, just below the groin ; additional (6) but less constant lateral whorls 

 may be located, one on each side, about midway between the axilla and the iliac crest and 

 somewhat beyond the outer border of the rectus muscle. 



These whorls, all paired except the first, apportion the entire surface of the body into 

 certain districts, each covered by the hairs proceeding from the corresponding vortex. The 

 whorl-districts, moreover, are irregularly subdivided into secondary areas by lines, the hair- 

 ranges (flumina pilorum), along which the hairs diverge in opposite directions. Additional lines, 

 the converging hair-ranges, mark the meeting of tracts pointing in different directions and in 

 places also assume a spiral course. In consequence of these peculiarities the body is covered 

 with an elaborate and intricate hair-pattern, that is most evident on the foetus towards the close 

 of gestation ; later in life the details of the pattern are uncertain owing to its partial effacement 

 by the constant rubbing of clothing. 



Structure. Each hair consists of two parts, the shaft, which projects beyond 

 the surface, and the root, which lies embedded obliquely within the skin, the deepest 

 part of the root expanding into a club-shaped thickening known as the bulb. The 

 root is covered with a double investment of epithelial cells, the inner and outer root- 

 sheaths, which, in turn, are surrounded by a connective tissue envelope, the theca. 

 The entire sac-like structure, consisting of the hair-root and its coverings, constitutes 

 the hair-follicle (folliculus pili). At the bottom of the latter, immediately beneath 

 the bulb, the wall of the follicle is pushed upward to give place to a projection of 

 connective tissue, the hair-papilla, which carries the capillary loops into close relation 

 with the cells most active in the production of the hair. 'Save in the case of the 

 finest hairs (lanugo), which are limited to the corium, the hair-follicles traverse the 

 latter and end at varying levels within the fat-laden subdermal layer (panniculus 

 adiposus). In a general way the follicle may be regarded as a narrow tubular invagi- 

 nation of the epidermis, at the bottom of which the hair is implanted and from the 

 entrance of which the shaft projects. The most contracted part of the follicle, the 

 neck, lies at the deeper end of the relatively wide funnel-shaped entrance to the sac. 

 Closely associated with the hair-follicle, which they often surround, are the sebaceous 

 glands that pour their oily secretion at the upper third of the follicle into the space 

 between the shaft and the wall of the sac. 



The Hair-Shaft. In many thick hairs, but by no means in all, three parts 

 can be distinguished the cuticle, the cortex and the medulla. The latter, however, 

 is usually wanting in hairs of ordinary diameter, being often also absent in those of 

 large size. 



1 Frederic : Zeitschr. f. Morph. u. Anthropo!., Bd. ix., 1906. 



