THE HAIR. 



257 



Fig. 163. 



whole diameter of the hair ; being both relatively and absolutely 

 thickest in short, thick hairs, and thinnest in the lanugo and the 

 hairs of the head. It presents 

 a rounded or flattened figure in 

 transverse sections. (Figs. 161, a, 

 and 163.) Very rarely the me- 

 dulla is double throughout ; but 

 it is more frequently divided for 

 a distance into two trunks, which 

 soon unite again. In the Eo- 

 dentia (beaver, squirrel, &c.) the 

 medulla is divided by dissepi- 



ments ; in the musk-deer it con- 

 stitutes the entire hair, except a 



or flattened cells. 



Fig. 164. 



Cells of medulla of Rodentia, &c. A. Hair of 

 musk-deer, formed almost entirely of polygonal 

 cells. B. Hair of sable, showing large rounded 

 Very thin CUticle, and in the Sable cells in tlie interior, covered by imbricated scales 



its cells are very large. (Fig. 163.) 

 The hair of the bat and the 

 squirrel is shown by Fig. 164. 



B. The hair-sacs are flask-like 

 follicles, T J to ^V f an i ncn l n g 

 extending into the upper layers 

 only of the corium in case of 

 the finest hairs; about one-half 

 through it in case of those of 

 medium size ; and even through 

 to the subcutaneous areolar tissue 

 in case of the longest and strong- 

 est (the whiskers, on the head, 

 pubes, and axillae). They are 

 merely involutions of the skin, 

 at the bottom of which the \&\Y -papilla is situated. They have there- 

 fore an internal epithelium or root-sheath, and an external or fibrous 

 layer; the latter being continuous with the corium, and the former 

 with the epidermis. (Fig. 162, i, k, and e,f, g.) 



The root-sheath, or the epidermic investment of the hair-sac, is 

 continuous with the epidermis around the aperture of the sac, and 

 consists of two layers, an internal and an external. The external root- 

 sheath (g) is continuous with the Malpighian layer (rete mucosum) 

 of the skin, and rests on a distinct basement membrane; which, how- 

 ever, cannot be demonstrated between this and the internal fibrous 

 17 *- 



A. Small hair of squirrel. B. Large hair of squir- 

 rel, c. Hair of India bat. 



