Chap, v.] NUTRITION. 647 



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 con- 

 tractions. 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 mechanism 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 excessive, 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. 



§ 429. In a number of experiments it has been shewn that 

 injuries to, such as those caused by puncture or galvanic cautery, 

 or electrical stimulation of limited portions of the more cen- 

 tral 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 shewn, by the increase of 

 metabolism, increased production of carbonic acid and increased 

 consumption of oxygen, as well as by direct calorimetric obser- 

 vations, 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. 



§ 430. By regulative mechanisms of the kind just dis- 

 cussed the temperature of the warm-blooded animal is main- 

 tained within very narrow limits. In ordinary health the 

 temperature of 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 (-25° to 1*5°) higher; in the rectum it is still 

 higher (about -9°) than in the mouth. The temperature of 

 infants and children is slightly higher and much more sus- 

 ceptible of variation than that of adults, and after 40 years of 

 age the average maximum temperature (of health) is somewhat 

 lower than before that epoch. A diurnal variation, indepen- 

 dent of food or other circumstances, has been observed, the 



