RESPIRATION AND RELEASE OF ENERGY 121 



much below 37°C. Regulation works in both directions. When the 

 internal temperature falls to a certain degree, shivering is caused, and 

 heat is produced by the additional muscular movement. When the 

 temperature rises too far, there are several ways of checking it. Rapid 

 breathing serves to cool the lungs, and with them the whole body. More 

 blood flows to the skin; hence there is greater loss of heat by radiation. 

 And in man and horses, but not so much in many other mammals, sweat 

 exudes upon the surface, where its evaporation serves to lower the tem- 

 perature. In the dog there are no sweat glands except on the nose and 

 on the foot pads. In this animal rapid ventilation of the lungs in pant- 

 ing is the chief source of control; whatever cooling is caused by evapora- 

 tion occurs in the open mouth and on the lolling tongue. 



Regulation of temperature is governed by a nerve center in the 

 thalamus of the brain. When this center is warmed, the nerves going 

 to the blood vessels in the skin cause the latter to enlarge, and the sweat 

 glands are stimulated to excrete. On cooling the nerve center, these 

 actions are reversed, and muscle tension is increased, all of which leads 

 to a rise of temperature. 



References 



Carlson, A. J., and V. Johnson. The Machinery of the Body. The University 



of Chicago Press. (Chap. VI.) 

 Mitchell, P. H. Textbook of General Physiology. 3d Ed. McGraw-Hill Book 



Company, Inc. (Chap. XXI.) 



