316 HAND-BOOK OF PHYSIOLOGY. 



(speaking very generally) greater than that consumed by the same men 

 or animals in opposite conditions of climate and season. And the food, 

 which appears naturally adapted to the inhabitants of the coldest 

 climates, such as the several fatty and oily substances, abounds in carbon 

 and hydrogen, and is fitted to combine with the large quantities of oxy- 

 gen which, breathing cold dense air, they absorb from their lungs. 



(b.) By Exercise. In exercise, we have an important means of raising 

 the temperature of our bodies (p. 310). 



(c.) By Influence of the Nervous System. The influence of the nerv- 

 ous system in modifying the production of heat must be very important, 

 as upon nervous influence depends the amount of the metabolism of the 

 tissues. The experiments and observations which best illustrate it are 

 those showing, first, that when the supply of nervous influence to a part 

 is cut oft', the temperature of that part falls below its ordinary degree; 

 and, secondly, that when death is caused by severe injury to, or removal 

 of, the nervous centres, the temperature of the body rapidly falls, even 

 though artificial respiration be performed, the circulation maintained, 

 and to all appearance the ordinary chemical changes of the body be com- 

 pletely effected. It has been repeatedly noticed, that after division of 

 the nerves of a limb its temperature falls; and this diminution of heat 

 has been remarked still more plainly in limbs deprived of nervous influ- 

 ence by paralysis. 



With equal certainty, though less definitely, the influence of the 

 nervous system on the production of heat, is shown in the rapid and 

 momentary increase, of temperature, sometimes general, at other times 

 qpte local, which is observed in states of nervous excitement; in the 

 general increase*of warmth of the body, sometimes amounting to perspi- 

 ration, which is. excited by passions of the mind; in the sudden rush of 

 heat to* the face, which is not a mere sensation; and in the equally rapid 

 diminution of temperature in the depressing passions. But none of these 

 instances suffice to prove that heat is generated by mere nervous action, 

 independent of any chemical change; all are explicable, on the supposi- 

 tion that the nervous system alters, by its power of controlling the calibre 

 of the blood-vessels, the quantity of blood supplied to a part; while any 

 influence which the nervous system may have in the production of heat, 

 apart from this influence on^ the blood-vessels, is an indirect one, and is 

 derived, from, its power of causing such nutritive change in the tissues as 

 may, by involving the necessity of chemical action, involve the produc- 

 tion of heat. 



Inhibitory heat-centre. Whether a centre exists which regulates the 

 production of heat in warm-blooded animals, is still undecided. Experi- 

 ments have shown that exposure to cold at once increases the oxygen 

 taken in, and the carbonic acid given out, indicating an increase in the 

 activity of the metabolism of the tissues, but that in animals poisoned by 



