THE SKIN 307 



there has run down from the general surface of the body. Mules 

 and donkeys sweat with difficulty, and then principally at the 

 base of the ears. The ox sweats freely on the muzzle, and sweat- 

 ing even from the general surface of the body has occasionally 

 been observed. It has been said that sheep perspire, while it 

 is certain that both the dog and cat, especially the latter, sweat 

 freely on the footpads, as also on the muzzle, though not on 

 the general surface of the body. The sweating of the pig is 

 confined to the snout. 



The secretion of sweat is continuous. When excreted in small 

 amounts it evaporates as fast as it is formed, passing off as the 

 insensible vapour which is always rising from the surface of the 

 skin, and is known as ' insensible perspiration.' When the 

 secretion is rapid and copious or the surrounding atmospheric 

 conditions are unfavourable to its evaporation, it collects on 

 the skin as that visible fluid material which is ordinarily termed 

 * sweat.' Colin gives various numerical statements respecting 

 the insensible perspiration, from which we gather that 6-4 kilo- 

 grammes (14 pounds) of water probably represent this loss in 

 the horse for twenty-four hours. Much depends upon the 

 humidity and temperature of the atmosphere ; the drier and 

 hotter it is, within certain limits, the greater the insensible 

 perspiration. 



The amount of sweat secreted daily can only be roughly 

 determined ; there are many conditions which affect it, such as 

 the length of coat, nature of the work, and pace. Grandeau, 

 by estimating the total water consumed in the food and drink, 

 and that voided in the urine and faeces, arrived at the amount of 

 vapour passing away in the breath and perspiration. The mean 

 amount of water evaporated daily by these two channels, under 

 different conditions of work, was as follows : 



At rest - 29 kilogrammes ( 64 pounds). 



Walking exercise - - 39 „ ( 8*6 ,, ). 



At work walking - - 5'8 ,, (12*7 ,, ). 



Trotting - 60 „ (i3'4 „ )• 



At work trotting - - 94 ,, (20*6 ,, ). 



In each case the distance walked and trotted and the load 

 drawn were the same. It is unfortunate that we have no means 

 in the above experiments of determining the proportion which 

 the water of respiration bears to that of perspiration. 



Evaporation from the surface of the skin is a most important 

 source of loss of heat ; so marked is this in the horse that the 

 resulting fall in temperature may even carry it below the normal, 

 if the sweating be very profuse or the wetted area a large one. ' 



The compensating action existing between the kidneys and 

 skiji observed in man exists also in the horse — viz., when the 



