THE SKIN 



301 



though the temperature of the outside air may be at freezing- 

 point ; similarly, if taken from a warm to a cold locality, the 

 hair responds by becoming longer. Speaking generally, the 

 above statements are correct, but there are exceptions and 

 modifications. Some horses do not shed their coat after 

 passing into a warmer latitude ; the mechanism which regulates 

 the periodical shedding of hair refuses to respond to the 

 changed condition of affairs, so that in passing from north to 

 south of the Equator, with its reversal of seasons, the animal 

 may grow a summer coat in winter and vice versa for at least a 

 year after entering the new latitude. The permanent hair of 

 the body — viz., the 

 mane and tail — may 

 grow to almost any 

 length, but the tem- 

 porary hair of the sur- 

 face of the body only 

 grows to a definite 

 length. The full length 

 having been attained, 

 nothing will make it 

 grow longer, yet if the 

 horse be clipped, hair 

 at once grows rapidly, 

 but only to its original 

 length ; in other words, 

 everything is present 

 for the needful growth 

 to occur, but there is 

 a restraining influence 

 which determines the 

 length of hair accord- 

 ing to the season. 



Hair Streams. — This 

 term has been very 



aptly applied to the direction taken by the hair. Very little 

 observation shows that though the general trend of the stream 

 is obliquely downwards and backwards on the neck and trunk, 

 downwards on the face and limbs, and backwards on the 

 cheeks, that, nevertheless, the stream alters its course in an 

 interesting manner at various parts of the large surface, while 

 vortices are frequent. The number and position of the latter 

 are variable ; indeed, it has been considered that no two 

 horses are identically marked in this respect, and in Japan it 

 is employed for the identification of horses on much the same 

 lines as the thumb-print system among men. There are cer- 



Fig. 90. — Section of Horse's Skin, showing 

 the Casting Off of the Old Hair and 

 Growth of the New. 



It will be observed that both are emerging from 

 the same follicle. 



