90 PHYSIOLOGY OF FARM ANIMALS [CH. 



in poikilothermal or so-called cold-blooded' animals (e.g. reptiles 

 and amphibians) the temperature of the body is roughly speaking 

 the same as that of the outside air and varies with it. In 

 hibernating animals (e.g. hedgehogs) there is a marked fall in 

 the body temperature during the period of hibernation Avhen the 

 animal is in a state of deep sleep and complete inactivity. 



The average normal temperatures in man and the domestic 

 animals are as follows : 



These are the temperatures at or near the surface. In the 

 centre of the body the temperature is slightly higher. 



It is evident that since heat production is always going on 

 in the body there must be a corresponding heat loss if the tempera- 

 ture is to keep constant. It has been estimated that a horse 

 produces sufficient heat during rest to raise the temperature to 

 boiling point in less than two days. The heat loss which prevents 

 such a rise of temperature is regulated by certain special 

 mechanisms in the skin. 



The muscles are the chief seat of heat production. They 

 make up half or more than half of the body weight. During 

 rest the oxidation processes which give rise to heat are always 

 going on, and during activity the output of heat is still greater, 

 as can be proved directly by observing that the temperature of 

 a muscle rises as a consequence of contracting. Besides the 

 muscles, however, heat production is associated with oxidation 

 in all the organs, and the liver and other glands are sources of 

 much bodily heat. 



This constant production is compensated for by evaporation, 

 radiation and conduction which takes place over the surface of 

 the skin. The warmer the surface the greater is the heat loss, 

 and the more blood going to the surface, the greater the warmth 

 of the surface. The quantity of blood going to the surface is 

 regulated by the nervous system. Thus when the surrounding 

 air is warm, or when the bodily heat is increased by muscular 

 exertion, the skin becomes hot and the vessels are dilated. The 



