CH. XL.] NERVOUS CONTROL OF TEMPERATURE 605 



(1) Though in cold-blooded animals a rise or fall of the surround- 

 ing temperature causes respectively a rise and fall of their metabolic 

 activity, in a warm-blooded animal the effect is just the reverse. 

 Warmth from the exterior demands a diminished production of heat 

 in the interior, and vice versa. For exceptions, see p. 592. 



(2) That this is due to a reflex nervous impulse is supported by 

 the fact that a warm-blooded animal, when poisoned by curare, no 

 longer manifests its normal behaviour to external heat and cold, but 

 is affected in the same way as a cold-blooded animal. Section of 

 the medulla produces the same effects, as the nerve-channels, by 

 which the impulses travel, are severed. When curare is given, the 

 reflex chain is broken at its muscular end, the poison exerting its 

 influence on the end-plates, and causing a diminution of the chemical 

 tonus of the muscles. The centre of this thermotaxic reflex mechanism 

 must be situated somewhere above the spinal cord ; according to some 

 observers, in the optic thalamus. 



(3) The reflex mechanism is well exemplified in shivering ; here 

 the muscles are thrown into involuntary contraction, and so produce 

 more heat, as the result of the stimulation of the skin by cold. 



(4) Various injuries caused by accident, or purposely produced 

 by puncture, or cautery, or electrical stimulation of limited portions 

 of the more central portions of the brain, may give rise to great 

 increase of temperature, not accompanied by other marked symptoms. 



We thus see that the nervous system is intimately associated 

 with the regulation of the temperature of the body. There is at 

 least one there may be several centres associated in this action. 

 The centres receive afferent impulses from without ; they send out 

 efferent impulses by at least three sets of nerves : (1) the vaso-motor 

 nerves, (2) the secretory nerves of the sweat-glands, (3) trophic or 

 nutritional nerves. The first two sets of nerves, the vaso-motor and 

 the secretory, affect the regulation of temperature on the side of 

 discharge ; the third set on the side of production. 



The foregoing account of heat regulation does not take into account what after 

 ah 1 is, at any rate in man, a very important factor, namely, the voluntary and arti- 

 ficial means which he employs," such as various kinds of clothing suitable to the 

 climate, heating of rooms, and voluntary muscular exercise. 



