THE ENERGY OF THE BODY. 503 



tures diminishes the bodily metabolism. We can best explain these results 

 by supposing that, under normal conditions, the muscles, which as we have 

 seen contribute so largely to the total heat of the body, are placed, by means 

 of their motor nerves and the central nervous system, in some special con- 

 nection with the skin, so that a lowering of the temperature of the skin leads 

 to an increase, while a heightening of the temperature of the skin leads to a 

 decrease of the muscular metabolism. Further, the centre of this thermo- 

 taxic reflex mechanism appears to be placed somewhere in the nervous system 

 above the spinal cord. When urari is given, the reflex chain is broken at 

 its muscular end ; when the spinal cord is divided, the break is nearer the 

 centre. 



We may add that the muscular metabolism which thus helps to regulate 

 temperature need not involve visible muscular contractions. At the same 

 time, the heat given out by the muscles will be temporarily increased at 

 every contraction which may occur. Thus, the shivering which follows 

 exposure to cold distinctly helps to warm the body ; indeed, some observers 

 have been led to think that, in man, this visible effect of cold plays a more 

 important part in his heat regulation than the invisible actions which we 

 have just described. We may also add that the regulative nervous mechan- 

 ism may apparently be overborne by an exposure to too great heat or cold. 

 When, for instance, the cold to which the animal is exposed becomes exces- 

 sive, the reaction of the thermotaxic nervous system is powerless against the 

 direct action on the tissues of the depressing influences, and the metabolism, 

 together with the temperature, sinks. 



449. In a number of experiments it has been shown that injuries to, 

 such as those caused by puncture or galvanic cautery, or electrical stimula- 

 tion of, limited portions of the more central portions of the brain may give 

 rise to a great increase of the temperature of the body without producing 

 any other marked symptom. The increase is shown by the increase of met- 

 abolism, increased production of carbonic acid, and increased consumption 

 of oxygen, as well as by direct calorimetric observations, to be due to an 

 increased production of heat. This naturally suggests that the portions of 

 the brain in question contain the hypothetical heat centre just mentioned, 

 the lesion on stimulation exciting the centre to activity by direct action on 

 it, instead of in the usual reflex manner. The matter has not, however, as 

 yet been clearly worked out ; and indeed observers are not agreed as to the 

 exact parts of the brain injury to which, or stimulation of which, produces 

 the effect. 



450. By regulative mechanisms of the kind just discussed the tem- 

 perature of the warm-blooded animal is maintained within very narrow 

 limits. In ordinary health the temperature of a man varies between 36 

 and 38, the narrower limits being 36.25 and 37.5, when the thermometer 

 is placed in the axilla. In the mouth the reading of the thermometer is 

 somewhat (0.25 to 1.5) higher ; in the rectum it is still higher (about 

 0.9) than in the mouth. The temperature of infants and children is 

 slightly higher and much more susceptible of variation than that of adults, 

 and after forty years of age the average maximum temperature (of health) 

 is somewhat lower than before that epoch. A diurnal variation, independent 

 of food or other circumstances, has been observed, the maximum ranging 

 from 9 A.M. to 6 P.M. and the minimum from 11 P.M. to 3 A.M. Meals cause 

 sometimes a slight elevation, sometimes a slight depression, the direction of 

 the influence depending on the nature of the food alcohol seems always to 

 produce a fall. Exercise and variations of external temperature, within 

 ordinary limits, cause a very slight change, on account of the compensating 

 influences which have been discussed above. The rise from even active 



