METABOLISM 269 



glands and chiefly, on account of its great size, in the liver. 

 During active digestion the temperature of the blood coming 

 from the liver is distinctly higher than that of the blood 

 going to the organ, and, since the amount of blood passing 

 through, the organ is large, an appreciable amount of heat 

 is derived from it. The production in glands, however, is 

 trivial when compared with the production in muscle. 



Heat Regulation. 



In spite of wide fluctuations in rate of heat production 

 and heat loss, the body temperature varies within very 

 narrow limits. This constancy is maintained by two means, 

 called respectively the physical regulation, which controls 

 the rate of loss of heat, and the chemical regulation, which 

 controls the rate of production. 



Physical Regulation. — When heat production exceeds heat 

 loss, the resultant rise in body temperature is accompanied 

 by dilatation of the cutaneous vessels, so that more blood is 

 brought to the surface and the loss of heat increased. Along 

 with this an increased secretion of sweat occurs. 



Conversely, when heat loss exceeds heat production, 

 constriction of cutaneous vessels and decrease of sweat 

 secretion reduce the loss of heat. 



These adjustments in the skin to maintain the balance 

 between heat production and heat elimination are reflex 

 effects. 



In the dog, which has no sweat glands on the part of 

 the body covered by hair, increased elimination of heat is 

 brought about by a panting respiration that increases the 

 loss of heat by the respired air and by evaporation from the 

 respiratory passages. The mouth is held open and the tongue 

 allowed to hang out, so that as large a moist surface as 

 possible may be exposed to the air to allow cooling by 

 evaporation. In cattle increased elimination of heat by 

 sweating is small in amount, and their temperature is 

 therefore especially liable to rise with exercise. 



Chemical Regulation. — As the temperature of the environ- 

 ment of an animal falls, a point may be reached at which 

 even with loss of heat restricted to the utmost, heat pro- 



