378 A MANUAL OF VETERINARY PHYSIOLOGY 



2. By Evaporation from the skin the sweat is converted into 

 vapour and heat is lost, the rapidity of the process depending 

 on the humidity of the air and its rate of movement. If the 

 air be saturated, no evaporation occurs from the skin, and, 

 consequently, no heat is lost. The value of evaporation as a 

 source of heat-loss in the horse is considerable, probably higher 

 than the figure fixed for man — viz., 14-5 per cent, of the total — ■ 

 but no data are available. Evaporation is constantly occurring ; 

 when the amount of sweat is small it is evaporated as fast as 

 it is produced, and this is referred to in the chapter on the skin 

 as insensible perspiration. The sensible perspiration is that 

 which is not evaporated as rapidly as it is produced, and is the 

 source of a much greater loss of heat. 



3. Evaporation from the mouth and nostrils, warming of in- 

 spired air, and vapourising of water from the lungs. The former 

 is a very valuable means of heat loss in those animals which do 

 not sweat from the general surface of the skin ; the moist nose 

 and open mouth of the dog are good examples of the principle, 

 and in a much smaller degree the bedewed muzzle of the ox. 

 The warming of the inspired air and the vapourising of water 

 from the lungs are most important sources of heat loss in those 

 animals which do not sweat. The panting respirations of the 

 dog, and of cattle and sheep in ' show ' condition, are simply 

 a means of cooling the body by warming a larger volume of air, 

 and offering a moist surface for evaporation. No evaporation 

 can occur when the air is saturated with vapour. 



4. By the urine and faces a loss of heat is incurred in warming 

 the food and water to the temperature of the body. The amount 

 of loss thus brought about must be relatively considerable, 

 especially in winter ; the abstraction of heat after drinking may 

 be so great as to cause shivering ; experiment shows that drinking 

 a pailful of water at 50 F. may cause the body temperature of 

 the horse to fall 0-5° to 0-9° F. A diet of roots, containing as 

 they do 80 per cent, water, is a heavy source of heat loss with 

 cattle in winter, though both in the case of the water consumed 

 and the succulent food ingested, no actual loss of heat occurs 

 until these are excreted as urine and faeces. 



The heat lost by conduction, radiation, and evaporation, is 

 greater in small than in large animals, as small animals have a 

 relatively greater surface exposed in proportion to their body 

 weight (see Fig. 107). A dog of 66 pounds weight will lose 

 70/5 per cent, of his body heat by radiation and conduction, 

 and 20*5 per cent, by the evaporation of water; whereas a dog 

 weighing 8 pounds will lose 91 per cent, by radiation, etc., and 

 9 per cent, by water evaporation. 



Bulk of body is a safeguard in large animals against sudden 



