ANIMAL HEAT. 431 



maximum occurring late in the afternoon, from 5 to 7 o'clock, the minimum 

 early in the morning, from 4 to 7 o'clock. This diurnal variation in 

 the mean temperature is related to corresponding variations in many other 

 physiologic processess, and its causes are to be found in the ordinary habits 

 of life as regards the time of meals, periods of exercise, sleep, etc. 



Food and Drink. The ingestion of a hearty meal increases the tempera- 

 ture but slightly not more than 0.5 C. Insufficiency of food lowers 

 the temperature; total withdrawal of food, as in starvation, is followed by 

 a steady though slight decline, until just preceding the death of the animal, 

 when it falls abruptly to from 6 to 8 C. Cold drinks lower, hot drinks 

 raise the temperature. Food and drinks, however, only temporarily change 

 the mean temperature, and after a short period equilibrium is restored 

 through the activity of the heat-regulating mechanism. Alcoholic drinks 

 lower the temperature about 0.5 C. In large toxic doses in persons un- 

 accustomed to their use the temperature may be lowered several degrees. 

 This is attributed not to a diminution in heat-production, but rather to an 

 increase in heat-dissipation (Reichert) from increased action of the heart, 

 dilatation of the blood-vessels of the skin, and increased activity of the 

 sweat-glands. 



Exercise. The temperature may be raised by active muscular exercise 

 from i to 1.5 C. as a result of increased activity in chemic changes in the 

 muscles themselves. A rise beyond this point is prevented by the increased 

 activity of the circulatory apparatus, the removal of the heat to the surface, 

 and its rapid radiation. 



External Temperature. The external temperature influences but slightly 

 the mean temperature of the human body. In the tropic, as well as in the 

 arctic regions, notwithstanding the change in the temperature of the air, 

 the temperature of the body remains almost constant. The same is true for 

 the seasonal variations in the temperature of the temperate regions. 



THE SOURCE AND TOTAL QUANTITY OF HEAT PRODUCED. 



The Source of Heat. The immediate source of the body-heat is to be 

 found in the chemic changes which take place in all the tissues and organs 

 of the body. Each contraction of a muscle, each act of secretion, each 

 exhibition of nerve-force, is accompanied by the evolution of heat. The 

 chemic changes are for the most part of the nature of oxidations, the union 

 of oxygen with the elements, carbon and hydrogen, of the food principles 

 either before or after they have become constituents of the tissues. The 

 ultimate source of the body-heat is the latent or potential energy in the food 

 principles, which was absorbed from the sun's energy and stored up during 

 the growth of the vegetable world. In the metabolism of the animal body 

 the food principles are again reduced through oxidation, directly or 

 indirectly, to relatively simple bodies, such as urea, carbon dioxid, and 

 water, with a liberation of a large portion of their contained energy which 

 manifests itself as heat and mechanic motion. 



The Total Quantity. The total quantity of heat liberated in the body 

 daily may be approximately determined in at least two ways: (i) By deter- 

 mining experimentally the heat values of different food principles by direct 

 oxidation; (2) by collecting and measuring with a suitable apparatus, a cal- 



