380 A MANUAL OF VETERINARY PHYSIOLOGY 



75 per cent, less of the heat lost by black or grey, for white not 

 only absorbs less heat during the day, but loses less heat at 

 night. Grey horses are better suited to the tropics than any 

 other colour, and black horses least of all. The black skin of 

 the negro protects the deeper tissues from the sun's rays, from 

 which it might be argued that black horses in theory should 

 stand exposure to a tropical sun better than grey, but a grey 

 horse has a black skin, and the pigment prevents the rays from 

 penetrating. Loss of heat from the body surface can be experi- 

 mentally produced by varnishing the skin, so that the animal 

 dies from cold unless rolled up in cotton-wool (see p. 312). 



Influence of Heat and Cold. — A moderate degree of cold applied 

 to the external surface of the body increases the production of 

 heat, due to increased oxidations. This results, as we have seen 

 (p. 376), from reflex impulses discharged through the motor 

 nerves. At the same time the appetite is increased to meet the 

 extra demand, and foods rich in fat are instinctively sought after 

 by man. The same should be observed in the feeding of animals, 

 and an increase allowed in the food to meet the extra oxidations, 

 fat, if possible, forming part of it. The body will stand a con- 

 siderable degree of cold, but a continuous fall in external tempera- 

 ture cannot be withstood ; a point is reached where the rate of 

 heat production is below that of heat-loss, and the animal dies 

 from cold. Conversely the body is adjusted to withstand a 

 moderately high external temperature ; the heat of Arabia or 

 India, which renders surrounding objects, such as metal, too hot 

 to hold, and the very birds to sit with drooping wings and wide- 

 open mouth, is borne with impunity by the acclimatised horse ; 

 the heat-regulating mechanisms do not allow the external heat 

 to be stored up. A continuous rise in external temperature 

 cannot be long borne, and a point arrives when the heat kills, for 

 when the discharge of heat from the body ceases, as it does when 

 the surrounding air goes above a certain point, it becomes stored 

 up, and heat-stroke follows. A far higher temperature can be 

 borne when the air is dry than when moist, as evaporation from 

 the surface practically ceases in a moist atmosphere. Men have 

 been exposed to a temperature of 127 C. (260 F.) for a few 

 minutes without ill effects, and with no elevation of the body 

 temperature. When air has its humidity increased by 1 per cent, 

 it raises the loss by radiation and conduction 32 per cent., while 

 an increase of 25 per cent, in the humidity of the air is equal to 

 an increase of 2° C. in the external air. At a temperature of 

 31 C. (88° F.) in an atmosphere saturated with vapour the 

 regulating mechanism of man is exhausted, and a rise in body 

 temperature occurs. Horses taken from cold to hot latitudes 

 have to learn to compensate, and until they do so a febrile rise in 



