68 HUMAN PHYSIOLOGY. 



contractions of the muscular masses, and is probably due" to the increased 

 activity of chemical changes ; a rise beyond this point being prevented by 

 its diffusion to the surface, consequent on a more rapid circulation, radia- 

 tion, more rapid breathing, etc. 



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

 ture but slightly; an absence of food, as in starvation, produces a marked 

 decrease. Alcoholic drinks, in large amounts, in persons unaccustomed 

 to their use, cause a depression of the temperature, amounting from i to 

 2 F. Tea causes a slight elevation. 



External temperature. Long continued exposure to cold, especially if 

 the body is at rest, diminishes the temperature from i to 2 F., while 

 exposure to a great heat slightly increases it. 



Disease frequently causes a marked variation in the normal temperature 

 of the body, rising as high as 107 F. in typhoid fever, and 105 F. in 

 pneumonia; in cholera it falls as low as 80 F. Death usually occurs 

 when the heat remains high and persistent, from 106 to 110 F. ; the 

 increase of heat in disease is due to excessive production rather than to 

 diminished elimination. 



The source of heat is to be sought for in the chemical decompositions 

 and hydrations taking place during the general process of nutrition, and the 

 combustion of the carbonaceous compounds by the oxygen of the inspired 

 air; the amount of its production is in proportion to the activity of the 

 internal changes. 



Every contraction of a muscle, every act of secretion, each exhibition 

 of nerve force, is accompanied by a change in the chemical composition of 

 the tissues and an evolution of heat. The reduction of the disintegrated 

 tissues to their simplest form by oxidation ; the combination of the oxygen 

 of the inspired air with the carbon and hydrogen of the blood and tissues, 

 results in the formation of carbonic acid and water and the generation of a 

 large amount of heat. 



Certain elements of the food, particularly the non-nitrogenized substances, 

 undergo oxidation without taking part in the formation of the tissues, being 

 transformed into carbonic acid and water, and thus increase the sum of 

 heat in the body. 



Heat-producing Tissues. All the tissues of the body add to the 

 general amount of heat, according to the degree of their activity. But 

 special structures on account of their mass and the large amount of blood 

 they receive, are particularly to be regarded as heat producers ; e. g. : 



I. During mental activity the brain receives nearly one-fifth of the 



