ANIMAL HEAT. 457 



It has also been experimentally ascertained that the rate of heat 

 production varies somewhat with the kind of food taken, for example, 

 if sugar be added to the meal of meat given to the dog, the height of 

 maximum production is reached. It \vas always said that various nations 

 had found by experience what food was most suitable for the climate in 

 which they lived, and that such experience could be trusted to regulate 

 the quantity consumed. Although there have been no very conclusive 

 experiments to prove this view, yet it is a matter of general observation 

 that in northern climates and in colder seasons the quantity of food 

 taken is greater than in warmer climates or in warmer seasons. More- 

 over, the kind of food is different. For example, persons living in the 

 colder climates require much fat in order to produce the requisite 

 amount of heat. 



In exercise, we have an important means of raising the temperature of 

 our bodies, by it the muscular metabolism is increased, as is shown by 

 the increased output of carbon dioxide. 



Influence cf 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 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 off, the temperature of that part after a time falls below its ordi- 

 nary 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 circu- 

 lation maintained, and to all appearance the ordinary chemical changes 

 of the body be completely effected. It has been repeatedly noticed, that 

 after division of the nerves of a limb its temperature ultimately 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, 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. All of these examples, however, are explicable, on the suppo- 

 sition that the nervous system alters, by its power of controlling the 

 calibre of the blood-vessels, the quantity of blood supplied to a part. 



Apart, however, from this vaso-motor power of increasing the blood- 

 supply to internal organs, and to the tissues in general, by means of 

 which it is possible to increase their metabolism and so their production 

 of heat, there is evidence to suppose that there is another nervous appa- 



