REGULATION OF HEAT PRODUCTION. 963 



vessels in the skin are constricted, the supply of warm l^iood 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 car- 

 ried 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. Substances, such as 

 alcohol, which cause a dilatation of the skin vessels 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 con- 

 trolled through an acceleration of the breathing movements. The 

 greatly increased respirations in muscular activity must aid some- 

 what 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 instance — 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 famihar phenomenon. 

 Some of the more recent writers believe that the reflexes concerned 

 in the physical regulation act not simply by varying the sweat 

 formation and the amount of blood circulating in the skin, but 

 also in a deeper way by changing the concentration of the blood. 

 The general idea is that the w^ater of the body has a great capacity 

 for holding heat, and when the amount of water brought to the 

 surface of the body is increased the loss of heat is augmented, 

 while when the water is withdrawn into the interior the heat is 

 conserved. In line with this idea it is stated that the action of 

 heat on the skin not only increases the sweating and the cutaneous 

 blood-flow but also causes the blood to take up more water from 

 the tissues, thus diluting the blood and l^ringing a larger volume 

 of water to the surface. The effect of cold, on the contrary, is to 

 cause a concentration of the blood, that is, a passage of water 

 from the blood to the tissues, which together with the diminished 

 sweating and the constriction of the cutaneous vessels safeguards 

 the supply of body heat.* The physical regulation suffices to 

 protect the body temperature against variations in the outside 

 * For a further description and literature consult Barbour, ''Physiological 

 Reviews," 1, 295, 1921. 



