ANIMAL HEAT. 695 



units ; sheep, two and six-tenths heat-units ; the dog, four heat-units ; 

 and the sparrow, thirty-two heat-units. The cause of this difference may 

 be found in the fact that the smaller a sphere the greater is its superficial 

 area in comparison to its cubic contents. The same rule applies, also, to 

 the irregular form of the animal body, in which the smaller it is in pro- 

 portion to the weight of the animal the greater will be its superficial area. 

 It is, however, from the external surfaces that the greatest amount of 

 body heat is dissipated, and it is, therefore, seen why small animals lose 

 proportionately more heat than larger animals. 



Two different conditions are noted in reference to the heat which is 

 retained in the animal body, and which, therefore, causes the body tem- 

 perature. In the cold-blooded animal the temperature of the body is 

 not constant, but varies with that of the surrounding medium, rarely 

 being more than a few tenths of a degree above it. In the warm-blooded 

 animals, as in mammals and birds, on the other hand, the body tempera- 

 ture is, as a rule, higher than that of the surrounding medium, and is 

 independent of variations in the latter. The cause of this difference of 

 body heat lies in the difference in energy of the tissue changes. In the 

 cold-blooded animals the development of heat is so slight that this 

 amount of heat is at once given up to the cold atmosphere. If the ex- 

 ternal temperature be increased, this dissipation of heat is accordingly 

 diminished, and as a consequence part of the heat produced is retained 

 in the body and increases its temperature. On the other hand, if the 

 external temperature falls, the amount of heat dissipated is increased 

 and the body temperature falls. In animals with a constant body tem- 

 perature the amount of heat, on account of the greater energy of tissue 

 change, is so much greater that but a part alone is given up to the sur- 

 rounding medium. From the fact that the source of temperature is 

 found in the chemical changes occurring in the tissues, it is evident that the 

 development of heat will be greater in tissues in which such processes are 

 active than where they are sluggish. The temperature of the animal body 

 will, therefore, vary in different localities ; it will be greater in secreting 

 glands and contracting muscles ; it will be less where loss of heat is 

 favored, and, as a consequence, the exterior surfaces of the body will 

 possess a lower temperature than the inner cavities. 



In the lungs the blood gives up so much heat to the air that the 

 temperature of the blood in the left side of the heart is cooler than that 

 of the right, in spite of the development of heat which accompanies the 

 oxidation of haemoglobin. With this exception the arterial blood, as 

 being less exposed to loss of heat, ma}', as a rule, be stated to be warmer 

 than venous blood. 



The temperature of an organ will, therefore, depend upon the amount 

 of blood circulating through it. Under certain circumstances the venous 



