REGULATION OF HEAT PRODUCTION. 955 



The control through the vasomotor nerves is doubtless even 

 more important. The blood-vessels bring the warm blood to the 

 skin, where it loses its heat by conduction and especially by radia- 

 tion to the cooler air. When the surrounding air is much below the 

 temperature of the body the vasoconstrictor center is stimulated, 

 the blood-vessels in the skin are constricted, the supply of warm 

 blood to the skin is diminished, and therefore the amount of heat lost 

 is less. The reflex in this case may be attributed primarily to the 

 action of the cool air on the cold nerves of the skin. The impulses 

 carried by these fibers to the nerve centers stimulate the vasocon- 

 strictor center or that part of it controlling the vasomotor fibers 

 to the skin. On warm days, on the contrary, the blood-vessels 

 in the skin are dilated sometimes to an extreme extent, the 

 supply of warm blood is therefore increased, and more heat 

 is lost if the air is lower in temperature than the blood. The 

 reflex in this case may be regarded possibly as an inhibition of the 

 vasoconstrictor center through the warm nerves of the skin. Sub- 

 stances, such as alcohol, which cause a dilatation of the skin ves- 

 sels also increase the loss of body heat, in some cases to a sufficient 

 extent to lower the body temperature. To a smaller extent our 

 heat loss is controlled through an acceleration of the breathing 

 movements. The greatly increased respirations in muscular ac- 

 tivity must aid somewhat in eliminating the excess of heat produced, 

 although this factor must be much less important than the sweating 

 and the flushing of the skin which are produced reflexly during 

 muscular work. In some of the lower animals the dog, for in- 

 stance in which the sweat nerves are absent over most of the body 

 and in which the coat of hair interferes with the free loss by 

 radiation, it is found that the loss through the respiratory channel is 

 relatively more important. The panting of the dog is a familiar 

 phenomenon. Richet has studied this reflex upon dogs and has 

 designated the greatly accelerated breathing in warm weather or 

 after muscular exercise as thermic polypnea (according to Gad, 

 tachypnea). He assumes a special center for the reflex situated in 

 the medulla and acting through the respiratory center. It is a 

 curious fact, as shown by Langlois, that some reptiles exhibit a 

 similar reflex; when their body temperature is raised to 39 C. they 

 show a condition of marked polypnea (rapid breathing) the ap- 

 parent object of which is to augment the loss of heat from the 

 body. 



Regulation of Heat Production. Heat production is varied 

 in the body by increasing or decreasing the physiological oxida- 

 tions. This end is effected in part voluntarily by muscular exercise 

 or by taking food. Muscular contractions are attended by a 

 marked liberation of heat and it is a part of everyone's experience 



