384 VETERINARY PHYSIOLOGY 



Since the temperature of the skin is governed by the state 

 of the cutaneous vessels and the amount of sweat produced 

 by the state of the sweat glands, and since both of these are 

 under the control of the nervous system, the elimination of 

 heat from the skin is presided over by a nervous mechanism. 



B. Respiratory Passages. By conduction and radiation 

 and by evaporation from the respiratory passages, about 10 

 per cent, of the heat is got rid of in man. In the dog and 

 some other animals, the proportion of heat eliminated in this 

 way is considerably greater. 



C. Urine and Faeces. Since these are warmer than the 

 surrounding air, a certain amount of heat is lost through 

 them. The amount is small something less than 2 per cent, 

 of the whole. 



Temperature. In all higher animals, the loss of heat and 

 the production of heat are so nicely balanced that the tem- 

 perature of the body remains fairly constant under all con- 

 ditions. If an extra amount of heat is produced, say in 

 muscular exercise, it is at once eliminated by the skin, and, 

 if the body is exposed to a low temperature, loss of heat is 

 rapidly checked by contraction of the cutaneous vessels and 

 diminished activity of the sweat glands. 



Since heat is constantly being given off, the temperature at 

 the surface of the body is always lower than the temperature 

 in the interior. The temperature of the rectum may be taken 

 as a measure of the internal temperature. 



The mean daily temperature of the horse is 37 to 38 C. 



Under all normal conditions the temperature undergoes 

 only small variations, because the balance between production 

 of heat and elimination of heat is so well maintained. But 

 under abnormal conditions the balance is frequently upset. 

 Thus severe muscular work causes a temporary rise of tem- 

 perature, because heat elimination does not quite keep pace 

 with heat production. Exposure to very high temperatures 

 may cause a slight rise of temperature, while exposure to 

 excessive cold may cause a slight fall; but, unless in the 

 case of those unable to use their muscles, the change is small. 



While the higher " warm-blooded animals," mammals and 

 birds, maintain a constant temperature, the lower vertebrates, 



