92 PHYSIOLOGY OF FARM ANIMALS [CH, 



prolonged exertion without getting into a condition of distress, 

 whereas a horse can gallop for mUes. This is a point of some 

 importance when we consider the usual method of applying the 

 tuberculin test to cattle. Before injecting the tuberculin, the 

 temperature of the animal is taken frequently on several successive 

 days (but very often in practice on only one day). It is again 

 taken after injecting, and a rise of 2*5" F. consequent upon it is 

 regarded as denoting a reaction (that is to say, that the animal 

 is affected with tuberculosis which augmented by the tuberculin 

 injected causes a rise of temperature). It should be remembered 

 however in the light of what has just been stated that the rise of 

 temperature in animals so susceptible as cattle, may be due to 

 other and quite different causes, such as excitement or the 

 periodic occurrence of oestrus, and that a rise apparently associated 

 with an injection may be a coincidence. Furthermore, the 

 initial temperatures taken before an injection may be abnormally 

 high owing to the same or similar causes, and a genuinely tuber- 

 culous condition may pass unnoticed owing to there being little 

 appreciable rise of temperature following upon the injection. 



It is well known that moisture present in cold air assists in 

 heat conduction but that moisture in warm air hinders evapora- 

 tion. This is why a hot dry climate is so much less trying than 

 a hot moist one. In warm, moist, tropical climates it is difficult 

 for the heat-regulating mechanisms to cope with the extreme 

 conditions. 



Clipping or shearing is liable to throw a severe strain on the 

 heat-regulating apparatus, especially in cold moist weather. For 

 this reason a shepherd cannot be too careful not to shear his 

 sheep when the weather is unfavourable. On the other hand, 

 in very woolly or fat animals the accumulation of heat may be 

 such as to constitute a source of danger. 



It is interesting to note that in the pig the fat in the dermis is 

 normally well developed, for the pig has few hairs to withstand 

 the conduction of heat. 



In addition to the heat regulating mechanisms described above 

 there is some evidence that heat production within the body is 

 regulated by the outside temperature. Thus animals which are 

 exposed to cold tend to eat more, and so a greater quantity of 

 food is available for heat production. Starvation produces a 

 lowering of the temperature of the body, and the absorption of 



