100 HABIT AND INTELLIGENCE. [chap. 



involuntary convulsive struggles ; and the stimulation of 

 sometimes the motor nerves, unaccompanied by pain, is capable of 

 tion, producing the same effect.^ In such cases the vital energy 



is transformed into motor energy. But vital energy is 

 also capable of being exhausted in a perceptible degree by 

 nervous excitement, even when there is no motor action ; 

 as is shown by the fact that the sense of fatigue is pro- 

 duced by mental exertion, or by pain unaccompanied by 

 struggling, just as it is by muscular exertion. We should 

 probably be justified in concluding, even without more 

 sometimes direct evidence, that in such cases there must be a trans- 

 intoheat. formation of vital energy into heat. But there is direct 

 Valen- evidence of such transformation. Valentin has obtained a 

 perimwit ^^^^ °^ ^^ much as 11° centigrade in the temperature of the 

 sciatic plexus of a frog, by mechanical or electrical irrita- 

 tion of its spinal cord.^ It is quite impossible to say how 

 far this was due to pain : or, we ought rather to say, how 

 far it was accompanied by pain ; for the physical character 

 and results of nervous action appear to be exactly the same, 

 whether it is accompanied by sensation or not. 



It is a remarkable proof of the dependence of nervous 

 action on physical changes, that mental exertion causes the 

 brain to lose phosphorus, which is found in the excreta. 



I think I have given conclusive proof that nervous 

 action is capable of producing heat and raising the tem- 

 perature ; and, conversely, a lowering of nervous action has 

 a tendency to depress the temperature. The paralysed 

 Paralysed limb of a patient is usually somewhat colder than the 

 ^^ ^- healthy one. And a curious instance is on record, in which 

 a wound of the wrist, which must have affected a nerve," 

 produced partial insensibility in the forefinger, and lowered 

 its temperature 10° Fahr. below that of the thumb. ^ These 

 last two instances, however, only concern the local distri- 



1 As in those remarkable cases where the lower limhs have been deprived 

 of sensation and voluntary motion by injury to the spinal cord, and yet 

 have continued to respond, by convulsive kicks, to any such stimulus as 

 that of tickling the soles of the feet. 



* Medico-Chirurgical Review, January 1864. 



^ Carpenter's Human Physiology, p. 738. 



