ANIMAL HEAT. 323 



ties of oxygen which, breathing cold dense air, they absorb from their 

 lungs. 



(.) By Exercise. In exercise, we have an important means of rais- 

 ing the temperature of onr bodies. 



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

 nervous system in modifying the production of heat must be very 

 important, as upon nervous influence depends the amount of the metabo- 

 lism of the tissues. The experiments and observations which best illus- 

 trate it are those showing, first, that when the supply of nervous influ- 

 ence to a part is cut off, the temperature of that part after a time 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 completely effected. It has been repeatedly no- 

 ticed, that after division of the nerves of a limb its temperature ulti- 

 mately falls ; and this diminution of heat has been remarked still more 

 plainly in limbs deprived of nervous influence 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 

 quite 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 

 supposition that the nervous system alters, by it power of controlling the 

 calibre of the blood-vessels (p. 147), 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 production of heat. The existence of nerve-centres 

 and nerves which regulate animal heat (thermogenic) otherwise than by 

 their influence in trophic (nutritive) or vaso-motor changes, although by 

 many considered probable, is not yet proven. 



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

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

 periments 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 urari, exposure to cold diminishes both the metabolism and the 



