9 8 



MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



hair follicle. In both hair and follicle several layers may be 

 distinguished. In the follicle there is the basal layer and the 

 more superficial layers of the epidermis, without, however, any 

 clear differentiation of strata corneum and lucidum. At the 



bottom of the follicle (root of 

 the hair) these pass directly 

 into the hair itself, on the out- 

 side of which is the so-called 

 inner root-sheath (the walls of 

 the follicle forming the outer 

 root-sheath) composed of two 

 layers of cells (the outer called 

 Henle's layer ; the inner, Hux- 

 ley's layer). This inner root- 

 sheath does not reach the 

 external surface. The hair 

 proper consists of a central core 

 or medulla, around which are 

 several layers of cells, the cor- 

 tex, and on the outside is a cutic- 

 ular layer. The growing point 

 is at the bottom of the follicle, 

 where the basal layer of the 

 epidermis, by repeated cell di- 

 vision, adds to the base of the 

 hair. As the cells grow older 

 they become cornified, and the 



whole is gradually pushed out of the follicle by additions below. 

 Like feathers, hairs appear at first in well-defined tracts, 

 but later, by multiplication, this regularity is lost. Hairs are 

 least abundant in the whales, where they may be reduced to from 

 two to eight pairs in the neighborhood of the mouth, and even 

 these sometimes only occur during foetal life. Hairs may also 

 be enormously developed into organs of defence, as in the case 

 of the ' quills ' of the hedgehog and porcupines, while in the 

 case of the vibrissas (' whiskers ') near the mouth, they may 

 serve as sense organs (p. 69). 



Oil glands of the racemose type are usually found connected 



FIG. 107. Diagrammatic section of 

 a hair and its follicle, after Maurer. 

 C, cuticle ; CJ?, cortex ; E, epidermis ; 

 F, follicle; G, oil gland; HE, Henle's 

 layer ; HD, Huxley's layer ; M, me- 

 dulla ; JV, nerves ; P, hair papilla ; S, 

 outer root sheath (the inner root sheath 

 is composed of Henle's and Huxley's 

 layers) ; V, vein. 



