THE APPENDAGES OF THE SKIN. 907 



degree of sensibility, and enable them to play an important part as organs 

 of touch. 



The follicles of these large hairs show, between the second and third layere of 

 their wall, a vascular dilatation — a kind of erectile tissue — that probably plays a 

 part also in the exercise of touch (Leydig, Odenins, Paladino, etc.). 



Two sebaceous glatids, and a swoofh muscular fasciculus, are annexed to the 

 pilous follicle. The sebaceous glands — which have been already described — 

 open into the sheath of the hair by a small excretory canal, which traverses 

 the fibrillar walls of the follicle. The muscular fasciculus (jnusculus arrector 

 pili) is situated on the side to which the hair and its follicle are inclined ; it 

 arises from the superficial face of the derma, and terminates at the bottom of 

 the follicle, which it erects by contracting. When the fasciculi contract over a 

 wide surface, the extent of the skin is diminished, and the hairs are erected and 

 partially ejected from their follicles — producing the cutis anseriiia. 



(Next to the innermost layer of the corium of the hair is a layer of stratified 

 epithelium — the external root-sheatli ; and lining this, in the upper part of the 

 follicle, is the internal root-sheath, representing the stratum corneum. This 

 inner root-sheath divides, near the orifices of the sebaceous glands, into two 

 layers — Henle's sheath being the outer, and Huxley's sheath the inner. 



The formation of a hair is identical with tlie formation of the epidermis 

 by the papillary layer of the derma. The capillary plexus of the follicle throws 

 out plastic lymph which is converted into granules, then into cells, which become 

 elongated into fibres. The cells that are to form the surface of the hair, are 

 converted into flat scales that enclose the fibrous structure of the interior. As 

 these are successively produced, they overlap those previously formed, and give 

 rise to the waving lines seen on the circumference of the hair ; this overlapping 

 also causes the roughness experienced in drawing a hair between the fingers from 

 its point to the bulb. The latter is the newly formed part of the hair, its 

 expanded form being due to the greater bulk of the fresh cells. 



The colour of the hair is very varied in animals, ranging from black to 

 white, red and brown, with all the intervening shades. The tint also changes 

 at different periods of life, being sometimes altogether altered between the 

 juvenile and adult periods ; dark-coloured Horses becoming light-coloured as 

 age advances. Besides, it is never uniform in the same animal ; black Hoi-ses 

 not unfrequently having white patches and diverse tints, with other dissimilari- 

 ties. The disease termed " melanosis " is very common in old white Horses 

 which were previously grey, and is supposed to be due to the localization of the 

 black pigment at certain limited points. The hair grows according to the 

 climate, seasons, food, etc., and varies with the species and breed. The coat in 

 every animal is shed at certain times, and is replaced by new hairs. 



The hair preserves the skin from unhealthy external influences — wet and 

 cold for example. It is a bad conductor of heat, and therefore keeps the body 

 warm. The tentacula are very useful as tactile organs ; while the mane, fore- 

 lock, and tail keep away insects, and the long hairs of the fetlock and pastern 

 protect these parts from the injurious effects of cold and wet, and the action of 

 foreign bodies.) 



Homy Productions. 



The horny tissues form several groups. The first comprises the horns of 

 Ruminants ; the second, the so-called chestnuts of Solipeds ; the third, the pro- 



