354 ANATOMY FOR NURSES [Chap. XVIII 



In this way the whole body is warmed in somewhat the same 

 way as we warm a house, the warm blood in the blood-vessels 

 heating the tissues, as the hot water in the hot-water pipes heats 

 the rooms in water-heated dwellings. 



THE REGULATION OF HEAT 



The maintenance of the normal temperature of the body is due : 

 (1) to the control exerted by the nervous system, (2) to the regu- 

 lation of muscular exercise and diet, (3) to the use of clothing, and 

 (4) the use of hot and cold baths. 



Control of nervous system. — As the amount of heat lost through 

 the skin and lungs as well as the metabolism taking place in the 

 body is under the control of the nervous system, it follows that this 

 control is preeminently important. It is effected by means of the 

 heat centres, the sensory nerves, the sweat nerves, and the vaso- 

 motor nerves of the skin. 



Heat regulation by the skin. — When the external temperature 

 is high, tlie nerve-endings which respond to heat are stimulated, 

 and these impulses are transmitted over sensory nerves to the 

 nerve centres controlling the motor nerves of the sweat-glands. 

 The motor nerves stimulate the activity of the sweat-glands, and 

 an increased amount of sweat is poured out upon the surface of 

 the body. An increased amount of heat is required to vaporize 

 this sweat, and thus heat is lost. Excessive humidity interferes 

 with the evaporation of water, and thus interferes with the loss 

 of heat ; hence the discomfort experienced on hot, humid days. 



The sensory nerves which are stimulated by heat not only trans- 

 mit impulses that stimulate the sweat-glands to activity, but at 

 the same time transmit impulses that result in the depression of 

 the vaso-constrictor nerves of the arterioles of the skin. In con- 

 sequence the arterioles dilate and more blood is sent to the surface 

 to be cooled. When the external temperature is low, the sensory 

 nerve endings which are stimulated by cold transmit impulses 

 which result in stimulation of the vaso-constrictors, and conse- 

 quent contraction of the arterioles of the skin. This lessens the 

 amount of blood in the skin arterioles, and lessens the amount 

 of heat lost. 



Heat regulation by respiration. — The stimulation of the sen- 

 sory nerves of the skin that are affected by cold influences the 



