THE APPENDAGES OF THE SKIN 



763 



mammals, and some parts which appear at first sij>;lit to be bare are found on close 

 inspection to be provided with sparse and very fine hair. The hairs are constantly 

 being shed and replaced, but at certain periods in the horse, for example, they fall 

 out in great numbers, constituting the shedding of the coat. It is customary to 

 distinguish the ordinary hairs (the coat), which determine the color of the animal, 

 from the special varieties found in certain places. Among the latter an^ the long 

 tactile hairs about the lips, nostrils, and ejTs; the eyelashes or cilia; the tragi of 

 the external ear; and the vibrissae of the nostrils. Other special features will be 

 noted in the discussion of the skin of the various s])eci(^s. The hairs are directed 

 in such a way as to form more or less definite hair-streams (Flumina pilorum), 

 and at certain ])oints these converge to form vortices (Vortices pilorum). 



The part of the hair above the surface of the skin is the shaft (Scai)us pili), 

 while the root (Hadix pili) is embedded in a depression termed the hair-follicle 



Fig. .572. — Lateral View of Horse to Show Hair-streams and Vortices. 



fill' Ki'mstler.) 



(After Ellenberger-Baum, Anat. 



(Folliculus pili). A vascular papilla (Pa])illa pili) projects up in the fundus of the 

 follicle and is capped by the expanded end of the root, the bulb of the hair (Bulbus 

 pili). The hair-follicles extend obliquely into the corium to a varying depth; in 

 the case of the long tactile hairs they reach to the underlying muscle. Most of the 

 follicles have attached to them small unstriped muscles known as the arrectores 

 pilorum; thes(^ are attached at an acute angle to the under side of the deep part 

 of the follicle, and their contraction causes erection of the hair and compression of 

 the sebaceous glands, one or more of which open into the follicle. 



The hairs are composed of (epidermal cells, and consist from without inward of 

 three parts. The cuticle is comi)osed of horny, scale-like cells which overlap like 

 slates on a roof. Th(^ cortex consists of horny fusiform cells which are packed close 



