HAIR. 



G17 



487 



and in which are distinguished the e root,' the f stem,' and the 

 6 point.' 



The root is softer and lighter in colour than the stem, 1 is con- 

 tained in a canal of the skin or sheath, fig. 487, e, and expands at 

 the implanted end into the ' knob.' This part during the growth 

 of the hair has a conical cavity inclosing the f bulb,' ib. f, which 

 forms the ' pith ;' from its base there is reflected upon the ' knob ' 

 a capsular layer of cells which forms the i crust ; ' this layer is con- 

 tinued to near the outlet of the sheath ; 

 it consists of two or more layers of 

 cells, the outermost of which have 

 generally lost their ( nuclei.' The 

 proper tunic of the sheath is ' derm,' 

 lined by epiderm continuous with the 

 cuticle, which accordingly, when shed, 

 usually brings away the hairs. In the 

 dermic part there is a vascular and a 

 hyaline layer; the latter ceasing with 

 the capsular part of the hair's matrix. 



Two sebaceous glands, ib. A, usually 

 open into the hair-sheath; and one 

 or more delicate muscles, ib. g, of 

 unstriped fibre, pass from the harder 

 superficies of the derm to be inserted 

 into the capsule beneath the glands ; these are mainly concerned 

 in raising the hairs. 



Hairs, like teeth, are of two kinds as regards growth ; one 

 temporary, the other persistent. The former are shed and suc- 

 ceeded by new hair, usually once a year; the latter have persistent 

 bulbs and perennial growth. The body-hair of the Horse is an 

 example of the first kind, the hair of the mane and tail of the 

 second kind. In many Mammals there are two kinds of hair, 

 according to form, length, and structure ; one short, fine, more or 

 less curled, and mostly hidden by the longer, coarser, and straighter 

 kind, which is sometimes called the external coat, albeit the roots 

 sink deeper into the derm than do those of the internal coat, 

 usually called * fur.' 



These two kinds of hair — inner and outer — are most distinctly 

 as well as abundantly shown in arctic and aquatic quadrupeds, 

 (ermine, sable, beaver, and the seal-tribe), especially in the young 

 state, when the heat-fcrming power is weak. In some species of 



1 The contrast is striking in the hair of the Ornithorhynchns, in which the brown 

 tint is confined to the expanded terminal part of the hair. 



Section of skin tfith hair-matrices. 



