602 THE CENTRAL NERVOUS SYSTEM. [CH. XLI. 



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 pro- 

 duce 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 : (i) 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 all is, at any rate in man, a very important factor, namely, the voluntary 

 and artificial means which he employs, such as various kinds of clothing 

 suitable to the climate, heating of rooms, and voluntary muscular exercise. 



CHAPTER XLI. 



THE CENTRAL NERVOUS SYSTEM. 



WE already know sufficient from our preliminary study of 

 nerve-centres to be aware that the central nervous system is 

 divided into the two main parts called the brain and spinal cord. 

 We now return to the subject, and must enter into the somewhat 

 complicated details of the construction and mode of action of 

 these parts. 



Fig. 449 shows the general arrangement of the cerebro-spinal 

 axis, and some anatomical details concerning the membranes that 

 envelope the brain and cord may here conveniently be added. 



