THE HAIRS. 323 



The end-organs of Ruffini resemble the sensory terminations in tendons 

 (page 85) and lie within the deeper parts of the corium, often associated 

 with the Pacinian bodies. 



The mode of ending of the nerves supplying the hairs and sweat-glands 

 will be described in connection with those structures (pages 328 and 336). 



THE HAIRS. 



The appendages of the skin the hairs, nails, and cutaneous glands are 

 all specializations of the epidermis ; they are, therefore, exclusively of ecto- 

 dermic origin. 



The hairs are present over almost the entire body, the few localities in 

 which they are absent being the flexor surface of the hands and feet, the 

 extensor aspect of the terminal segment of the fingers and toes, the inner 

 surface of the prepuce and of the nymphae and the glans penis and clitoridis. 

 With the exception of those regions in which the growth is sufficiently long 

 to constitute a complete covering, the hairs are for the most part short and 

 scattered, although subject to great individual variation. The closest set 

 hairs are on the scalp, on the top of the head numbering from 300-320, and 

 in the occipital and frontal regions from 200-240 per square centimeter. On 

 the chin they number about 45, on the mons pubis 35, on the extensor sur- 

 face of the forearm 24 and on the back of the hand 18 for like areas. Even 

 where their distribution is seemingly uniform, close inspection shows the 

 hairs to be arranged in groups of from two to five. 



In their thickness the hairs show much variation, not only in different 

 races, individuals and regions, but also in the same person and part of the 

 body, as on the scalp where fine and coarse hairs may lie side by side. 

 The thickest scalp-hairs have a diameter of 162^ /j. and the finest one of 10 p. 

 with all intermediate sizes. The hairs of the beard vary from 100-200 /*, 

 and those on the pubes from 50-135 /*. In a general way, hairs of light 

 color are finer than dark ones. On attaining their full growth without 

 mutilation, hairs do not possess a uniform thickness throughout their length, 

 since they diminish not only towards the tip, where the shaft ends in a point, 

 but also towards the root. This feature is most evident in short hairs, as in 

 those of the eyebrows. The straight and curly varieties of hair depend 

 chiefly upon differences in the curvature of the follicle 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 les's bent and are flattened or grooved, 

 with corresponding oval, reniform, or irregularly triangular outlines when 

 transversely cut. 



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 con- 

 nective tissue envelope, the theca. The entire sac-like structure, consisting 

 of the hair-root and its coverings, constitutes the hair-follicle. At the 

 bottom of the latter, immediately beneath the bulb, the wall of the follicle is 

 pushed upwards 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 



