ANIMAL HEAT. 597 



in general. In fever, both heat-production and heat-dissipation are generally 

 increased, the former being affected more than the latter, so that the bodily 

 temperature rises. In some forms of fever the rise of temperature is essentially 

 due to diminished heat-dissipation. 



D. THE HEAT-MECHANISM. 



The heat-mechanism consists of two fundamental parts, one being concerned 

 in heat-production, and the other in heat-dissipation. Heat-production is 

 briefly expressed as thermogenesis ; and heat-dissipation, as thermolysis. The 

 operations of these mechanisms are so intimately related that fluctuations in the 

 activity of one are rapidly compensated for by reciprocal changes in the other, 

 so that under normal conditions heat-production and heat-dissipation so nearly 

 balance that the mean bodily temperature is maintained within narrow limits. 



The regulation of the relations between heat-production and heat-dissipation 

 is termed thermotaxis, which regulation may be effected by alterations in either 

 thermogenesis or thermolysis. 



The Mechanism concerned in Thermogenesis. The portion of the heat- 

 mechanism concerned in heat-production consists of (1) thermogenic tissues, 

 (2) thermogenic nerves, and (3) thermogenic centres. 



The Thermogenic Tissues. Almost if not every tissue of the body may be 

 regarded as being a heat-producing structure. The very fact that oxidative 

 processes lie at the bottom of all forms of vital activity, and that heat-produc- 

 tion is a concomitant of oxidation, leads inevitably to the conclusion that as 

 long as cells possess life they must produce heat. There are, however, certain 

 of the bodily structures, especially the skeletal muscles and the glands, which 

 are exceptionally active as heat-producers. Indeed, in the case of the skeletal 

 muscles the heat-producing processes are of such a character as to justify the 

 belief that with them thermogeuesis is a specific function, because heat is pro- 

 duced not merely as an incidental product of activity but as a specific product. 

 When a muscle contracts, heat is evolved as an incident of the performance of 

 work, and when it is at rest heat is produced not only as an incident of growth 

 and repair but as the result of a specific act. This latter is proved by the fact 

 that when the muscles have been in a state of prolonged rest, when the chemi- 

 cal changes concerned in growth and in repair of waste are inactive, heat-pro- 

 duction continues to a marked degree. Moreover, the quantity which is pro- 

 duced varies with the immediate needs of the economy and bears a reciprocal 

 relationship to the quantity of heat formed in other structures, 1 and is regulated 

 apparently by specific nerve-centres. 



When the muscles are contracting less than one-fifth of the energy appears 

 as work, and more than four-fifths as heat. The contractions of the heart also 

 furnish an appreciable percentage of heat as an accompaniment of contraction ; 

 and considerable heat is formed indirectly by the resistance offered by the 

 the blood-vessel walls to the blood current. Indeed, the entire work of the 

 heart becomes converted into heat, representing from 5 to 10 per cent, of the 

 1 Kiibner: Sitzungsberichte d. konigl. Bayer. Akad. der Wissenschaften, 1885, Heft 4. 



