304 A MANUAL OF VETERINARY PHYSIOLOGY 



ferment, and, in accordance with the systematic nomenclature 

 now used, is therefore known as tyrosinase. A further fact 

 of extreme interest is that the colour of the pigment formed from 

 tyrosine corresponds to the colour of the animal^from whose 

 skin the active extract is made. Black pigments are produced 

 when animals are used whose skin contains black pigment, and 

 yellow substances are obtained when the skin contains orange 

 pigment. 



Colours of Horses. — The body colour of animals is protective ; 

 whatever the tint may have been in the ancestors of horses, it 

 is intended to help in the struggle for existence. It is probable 

 that the colour was a dun ; far more protective colouring, how- 

 ever, is afforded another species of equine in the form of bars. 

 The zebra under certain conditions of light is invisible. With 

 the exception of black and grey horses, which are liable to turn 

 grey or white, all other colours are practically permanent even 

 to old age. We do know, however, that injuries to the skin of 

 horses, even of a slight character, are commonly followed by a 

 growth of perfectly white hair, which never regains its pigment. 

 In these cases the skin also loses its pigment ; pigment granules 

 are not reproduced in injured skin. The connection between 

 body colour and constitution is a physiological question to 

 which, unfortunately, no answer can be given. Colour and 

 temperament may go hand in hand ; other connections are not 

 less difficult to explain. Chestnut horses are frequently ex- 

 citable, nervous, and irritable ; all horses of light colour, no 

 matter what it may be, are, as a rule, wanting in stamina and 

 constitution. The ' mealy ' bay is typical of a bad colour and 

 almost invariably of a bad horse. Grey horses are not popular, 

 they give too much trouble to keep clean, and show every stain, 

 yet in the East such coloured horses are capable of great fatigue, 

 and possess remarkable stamina, provided that the skin is 

 pigmented. A grey horse with a pink skin is an albino, and is 

 worthless for hard work. For work and hard constitution a 

 roan is difficult to beat, especially a red roan, yet the colour is 

 not popular. The ' softest ' horse is the black, and on this point 

 there appears to be no difference of opinion. 



It is seldom that the colour of a horse is not broken by the 

 introduction of white ; this is common on the face and limbs, 

 and in days gone by a star on the forehead was so prized that 

 one was created if it happened to be absent. On the limbs the 

 amount of white is variable ; no exception is taken to it if limited, 

 but foUr white legs are not only unsightly but a source of weak- 

 ness. White legs are liable to • chap ' in winter, for white on 

 the limbs of a coloured horse is associated with a pink skin ; 

 such a skin is readily affected if washed and left wet. 



