7200 



PHYSIOLOGY OF THE NERVOUS SYSTEM. 



narcotic substances ; or, on the other hand, we 

 may unduly excite them by applying a strong 

 solution of strychnia. The contact of a solid 

 body with a nerve may irritate and keep up a 

 continual state of excitement, if it do not destroy 

 its properties. A spiculum of bone, in contact 

 with nervous fibres, is often the cause of the 

 severest forms of neuralgia; inflammation may 

 produce like effects. Various physical agents 

 may produce similar consequences. Thebenumb- 

 ing influence of cold is explained in this way. Ex- 

 posure to a continuous draught of cold air is a 

 frequent cause of facial paralysis. The giving 

 way of a carious tooth will immediately occa- 

 sion toothache by exposing the nerves of its 

 pulp to the irritating influence of the air, or of 

 the fluids of the mouth. And undue heat is 

 likewise injurious to the physical constitution, 

 and, therefore, to the action of nerves. These 

 facts are of great interest in reference to the 

 pathology of nervous diseases, and suggest that 

 the attraction of a morbid material in the blood 

 to a nerve or set of nerves, or to that part of the 

 nervous centre in which such nerves may be 

 implanted, may afford satisfactory explanation 

 of many obscure phenomena of nerves of sen- 

 sation. 



The organic change, whatever be its intrinsic 

 nature, which stimuli, whether mental or phy- 

 sical, produce in a nerve, developes that won- 

 derful power long known to physiologists by 

 the name vis nervosa, the nervous force. This 

 force is more or less engaged in all the func- 

 tions of the body, whether organic or animal. 

 In the former its office is to regulate, control, 

 and harmonize ; in the latter it is the main- 

 spring of action without which none of the 

 phenomena can take place. It is the natural ex- 

 citantofmuscularmotion, and the display of that 

 wondrous powerdepends upon its energy ; with- 

 out vigour in the developementand application 

 of the nervous force, a well-formed muscular 

 system would be of little use, for it would 

 quickly suffer in its nutrition if deprived of that 

 exercise which is essential to it. 



In the various combinations of thought 

 which take place in the exercise of the intellect, 

 there can be no doubt that the nervous force is 

 called into play in the hemispheres of the 

 brain. Here the stimulus is mental ; the inde- 

 pendent operations of the mind excite the ac- 

 tion of the appropriate fibres of the brain, and 

 the developement of the nervous force in the 

 brain immediately succeeds the intellectual 

 workings. It is thus that we explain the 

 bodily exhaustion which mental labour in- 

 duces; and thus, too, we can understand the 

 giving way of the brain the inducement of 

 cerebral disease under the incessant wear and 

 tear to which men of great intellectual powers 

 expose it. On the other hand, physical changes 

 in the brain, of a kind different from those 

 which are normal to it, the circulation of too 

 much, or too little, or of a morbid blood, may 

 excite mental phenomena in an irregular way 

 and give rise to delirium or mania. 



Of the conditions ncccssan/ for the maintenance 

 of the power of developing the nervous force. 

 From what has been already stated, it is mani- 



fest that a healthy physical state of the nervous 

 matter, whether in the nerves or in the nervous 

 centres, constitutes the main condition neces- 

 sary for preserving in them the power of deve- 

 loping the nervous force. And as nerves will 

 not maintain their healthy nutrition unless they 

 be in union with the nervous centres, this union 

 becomes an important condition for the main- 

 tenance of this power in nerves. In the ner- 

 vous centres the nerves form a connexion with 

 the vesicular matter. We therefore infer that 

 this connexion of the fibrous and vesicular matter 

 is necessary for the exercise of the peculiar power 

 of nerves, because we know of no instance, 

 either in the human economy or in that of the 

 inferior creatures, in which the nervous power is 

 developed without this union. 



It is true that if a motor nerve be separated 

 from the nervous centre, its peripheral segment 

 will evince a susceptibility to stimuli, or, in 

 other words, it will retain the power of gene- 

 rating the nervous force for some time after the 

 separation. This is, however, only for a short 

 period, as the experiments of Longet distinctly 

 show. Longet cut out a portion of the sciatic 

 nerve in dogs, and irritated the lower segment 

 of the nerve on each succeeding day by means 

 of galvanism from a pile of twenty couples, and 

 by mechanical irritation. The nerve ceased to 

 be excitable on and after the fourth day, (" des 

 le quatrieme jour.")* These results, although 

 they appear to differ from those obtained by 

 Mu'ller and Sticker, and Steinruch, are not really 

 inconsistent with them. These observers, instead 

 of examining and irritating the lower segment 

 of the nerve each succeeding day after the sec- 

 tion, allowed it to remain for an arbitrary period 

 untouched, and then reopened the wound to try 

 the effect of stimulating the nerve. Thus Mu'ller 

 and Sticker waited eleven weeks in one rabbit, 

 five weeks in a second, and two months and a 

 half in a dog, and in all the cases found the 

 nerve inexcitable ; and Steinruch waited four 

 weeks, at which time he found that the power 

 of the nerve had disappeared. It is obvious 

 that there was nothing in any of these experi- 

 ments to cast a doubt on the possibility of the 

 nerve having lost its excitability at a much 

 earlier period after the section, and that the 

 selection of five or eight or eleven weeks, as the 

 period when to inquire whether the nerve re- 

 tained its excitability or not, was entirely arbi- 

 trary on the part of the experimenters. 



The rapidity with which a nerve loses its 

 power after it has been separated from the 

 nervous centres clearly denotes that connection 

 with the centre is a necessary condition for the 

 nutritive activity of nerves, and is, therefore, a 

 necessary condition for their functional activity, 

 or, in other words, for the full developement 

 of the nervous force under its appropriate sti- 

 muli. There are, however, other facts which, 

 inasmuch as they enhance the importance of 

 the vesicular matter in the manifestation of 

 nervous phenomena, give great weight to the 

 proposition under consideration. These are 



* Lancet, Rccherches Exper-mentales sur 1'Irrita- 

 bilitc Miisculaire : PExaminatcur Med. Dec. 1841. 



