THE APPENDAGES OF THE SKIN 



763 



mammals, and some parts which appear at first sight to be bare are found on close 

 insiwction 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 are the long 

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

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

 noted in the discussion of the skin of the various species. The hairs are directed 

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

 and at certain points these converge to form vortices (A'ortices pilorum). 



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

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



^ 



■*i5^- 



Fig. 572. 



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



fiir Kiinstler.) 



(After Ellenberger-Baum, Anat. 



(FoUiculus pili). A vascular papilla (Papilla 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 ; these 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 composed of horny, scale-like cells which overlap like 

 slates on a roof. The cortex consists of horny fusiform cells which are packed close 



