270 THE SOLIDS. 



In some rare instances two medullary canals have been 

 observed in the same hair. In the sable the medulla has a 

 distinctly cellular structure throughout.* 



FOLLICLE OF THE HAIK. 



Each hair is implanted in a distinct depression in the 

 dermis, the base of which especially is freely supplied with 

 nutrient vessels : this depression is also lined by an invagi- 

 nation of the epidermis, and which becomes ultimately the 

 outer layer of the sheath of the root of the hair as already 

 described. (See Plate XXVI. /#. 3.) 



Between this layer, however, and the shaft of the hair for 

 a short distance before it rises above the level of the skin, a 

 space, or cavity is left, into this space the canals of one or 

 more sebacious ducts generally open, and in it also entozoa 

 frequently develope themselves. 



It sometimes happens that two or more hairs are contained 

 in the same follicle (see Plate XXVI. fig. 3.) : in these 

 cases, however, each hair has a distinct modelling sheath. In 

 some animals the location of a number of hairs in one sheath 

 is the ordinary mode of arrangement ; in the pig, for ex- 

 ample, the hairs are usually thus associated in threes, as also 

 occasionally in man. 



The hair follicle or crypt is best seen by examining thin 

 vertical slices of the skin. 



The length of the hair follicle and the consequent depth 

 of implantation of the hair varies, but is often equal to the 

 twelfth or sixteenth of an inch ; the hairs of the head, of the 

 whiskers^ of the pubis, and of the axilla penetrate into the 

 subcutaneous cellular tissue ; those of the eyelids and ears to 

 the subjacent cartilages : the roots of hairs in general, how- 

 ever, do not penetrate beyond one half the depth of the 

 corium, in the substance of which they are buried. 



It is usually stated that the bottom of the follicle is oc- 



* The first accurate observations on hair were made by Hook, Mico- 

 graphia, 1667, Obs. 32. tab. v. fig. 2., and Leeuwenhoek, Opera, t. iv. p. 46. 



