236 INNERVATION. [CHAP. IX. 



way of a carious tooth occasion toothache, by exposing the nerves 

 of its pulp to the irritating action of the air, or of the fluids of the 

 mouth ! And heat is equally injurious to the physical constitution, 

 and, consequently, to the action, of nerves. 



The organic change, whatever be its intrinsic nature, which sti- 

 muli, whether mental or physical, produce in a nerve, developes 

 that wonderful power long known to physiologists by the name vis 

 nervosa, the nervous force. This force is more or less engaged in 

 the play of all the vital functions, whether organic or animal. In 

 the former its office is to regulate, control, and harmonize, as will be 

 hereafter explained ; in the latter, it is the main-spring of action, 

 without which none of the phenomena can take place. It is the 

 natural excitant of muscular motion, and the display of that won- 

 drous power depends upon its energy. Unless there were vigour 

 in the development and application of the nervous force, a well- 

 formed muscular system would be of little avail, for it would 

 quickly suffer in its nutrition if deprived of that exercise which is 

 so necessary to it. 



Although the workings of the mind are doubtless independent of 

 the body, experience convinces us that in those combinations of 

 thought which take place in the exercise of the intellect, the nervous 

 force is called into play in many a devious track throughout the in- 

 tricate structure of the brain. How else can we explain the bodily 

 exhaustion which mental labour induces? The brain often gives way, 

 like an overwrought machine, under the long-sustained exercise of a 

 vigorous intellectual effort ; and many a master-mind of the present 

 or a former age has, from this cause, ended his days " a driveller 

 and a show." A frequent indication of commencing disease in the 

 brain is the difficulty which the individual feels in " collecting his 

 thoughts," the loss of the power of combining his ideas, or im- 

 pairment of memory. How many might have been saved from 

 an early grave or the madhouse, had they taken in good time the 

 warning of impending danger which such symptoms afford ! The 

 delicate mechanism of the brain cannot bear up long against the 

 incessant wear and tear to which men of great intellectual powers 

 expose it, without frequent and prolonged periods of repose. The 

 precocious exercise of the intellect in childhood is frequently 

 prejudicial to its acquiring vigour in manhood, for the too early 

 employment of the brain impairs its organization and favours the 

 development of disease. Emotion, when suddenly or strongly excited 

 or unduly prolonged, is most destructive to the proper texture of 

 the brain, and to the operations of the mind. Our lunatic hospitals 



