THE PRODUCTION AND DISSIPATION OF HEAT 1097 



body-temperature in different ways. Such agents as alcohol lower 

 it, because they stimulate the circulation and dilate the peripheral 

 blood-vessels. Both these changes, therefore, favor heat-dissipation. 

 The anesthetics and narcotics also lower it, because they depress 

 the oxidations and relax the blood-vessels. Strychnin, cocain and 

 nicotin increase it. 



The Regulation of the Body-temperature. Thermotaxis. — The 

 mere fact that the homoiothermal animals are able to retain a rather 

 uniform temperature in spite of their inconstant rate of heat-produc- 

 tion and almost incessant variations in the surrounding mediimi, 

 proves beyond a doubt that they must be in possession of a mechanism 

 whereby their body-temperature is regulated. Clearly, a constant 

 body-temperature can only be obtained if the dissipation and pro- 

 duction of heat are accurately balanced, and hence, any change there- 

 in must show that one of these factors is more powerful than the 

 other. Thus, a rise in the body-temperature may be due either to a 

 greater heat-production or to a diminished dissipation, or both. 

 Quite similarly, a lowering of the body-temperature may arise either 

 in consequence of a lessened heat-production or an increased heat- 

 dissipation, or both. In many cases, however, it is quite impossible 

 to state definitely which of these two factors is at fault unless the 

 fundamental cause of the variation is known. Thus, it is commonly 

 held that fever is due to a greater production of heat, although it 

 will be shown later on, that the heat-dissipating mechanism, consisting 

 of the vasomotor apparatus and the sweat-glands, is also deranged at 

 this time. 



It will be seen, therefore, that living animals behave very differ- 

 ently from dead animals, because the latter absorb and lose heat 

 somewhat in the manner of inorganic bodies It is a matter of common 

 observation that inanimate material may be artificially warmed and 

 cooled in accordance with its thermotactic properties. The plants 

 occupy an intermediate position, because they possess certain quali- 

 ties which allow them in a slight measure to resist outside thermic 

 influences. 



Heat-production. Thermogenesis. — Heat is a form of energy. 

 It is not matter, but merely a peculiar state of matter, because, in 

 accordance with the undulatory theory, the heat of a body is due to 

 an extremely rapid oscillation or vibration of its molecular constituents. 

 Now, since heat-rays occur free in nature, it is only natural to suppose 

 that animals must receive some of this energy in the form of ordinary 

 radiations from the sun or from artificial media. The chief and ulti- 

 mate source of heat, however, lies in the potential energy of the food 

 and, in a slight measure, also in hydrolytic cleavage. In this connec- 

 tion attention should again be called to the fact that the thermogenic 

 power of the tissues differs greatly in accordance with their physiolog- 

 ical purpose as well as with the state of their activity. Now, since 

 muscular exercise leads to the evolution of a large amount of heat, it 



