952 NUTRITION AND HEAT REGULATION. 



what higher temperature than adults. The difference may amount 

 to half a degree or a degree centigrade — 37.6° C. in infants as com- 

 pared with 36.6° or 37.1° C. in the adult. It is known, also, that 

 the heat-regulating mechanism in infants and young children is not 

 so efficient as in adults, and that therefore febrile disturbances are 

 more easily excited in the former than in the latter. In the matter 

 of body temperature, as in so many other characteristics, aged peo- 

 ple show a tendency to revert to infantile conditions. Their tem- 

 perature, according to most observers, is shghtly higher than in 

 middle life. Among physiological conditions that influence the 

 body temperature, muscular work and meals, as stated above, have 

 the most positive effect. Marked muscular activity imphes a great 

 increase in the production of heat in the body and most observers 

 find that the initial result at least is a small rise in body tempera- 

 ture — a fact which indicates that the heat regulation is not perfect; 

 the excess of heat produced is not dissipated promptly. This effect 

 is naturally most noticeable in tropical climates. In the period of 

 rest following upon work, on the contrary, the body temperature 

 may fall, owing probably to the fact that more heat is lost through 

 the flushed skin than is produced within the body. In this matter of 

 the effect of muscular work individual variations are to be expected, 

 since the perfection of the heat-regulating mechanisms may vary 

 somewhat in different persons. Meals also cause a slight rise in 

 body temperature, which reaches its maximum about an hour and 

 a half after the ingestion of the food. The explanation in this case 

 also is to be found doubtless in a greater production of heat, due to 

 the increased metabolism set up by the food (specific dynamic 

 action, see p. 906). The excessive production of heat is not 

 compensated completely by a corresponding increase in the 

 heat dissipated.* It is sufficiently obvious, perhaps, from these 

 facts that the temperature as measured by the thermometer is a 

 balance between the amount of heat produced and the amount of 

 heat lost or dissipated. The thermometer alone gives us no cer- 

 tain indication of the quantity of heat produced in the body. A 

 temperature higher than normal, fever temperature, may be due 

 either to an excessive production of heat or to a deficient dissipa- 

 tion. To understand and control the processes by which the body 

 temperature is kept normal it is necessary to discover a means for 

 ascertaining at any time the actual quantities of heat produced 

 and dissipated, and the effect upon each factor of different normal 

 and pathological conditions. The method used for determining 

 the quantity of heat is designated as calorimetry. It is necessary, 

 therefore, to describe the principle and construction of calorimeters 



* For further details see Richet, "La chaleur animale," 1889; and Pem- 

 brey, "Animal Heat," Schaefer's "Texi^-book of Physiology," vol. i, 1898, 



