722B 



PHYSIOLOGY OF THE NERVOUS SYSTEM. 



2. A second hypothesis is that which accords 

 with the views of Muller and many other phy- 

 siologists of the present day, and likewise pro- 

 bably with those of Whytt. It assumes that 

 the fibres of sensation and volition proceed to 

 and from some part or parts of the intracranial 

 nervous mass, that every nerve-fibre in the body 

 is continued into the brain. Those which are 

 distributed to the trunk and extremities pass 

 along the spina! cord, separating from it with the 

 various roots of the nerves, and in their course 

 within the spine mingling more or less with the 

 vesicular matter of the cord. There are, accord- 

 ing to this hypothesis, no other fibres but these, 

 (save the commissural,) and they are sufficient 

 to manifest the physical as well as the mental 

 acts. Nerves of sensation are capable of ex- 

 citing nerves of motion which are in their vici- 

 nity; and they may produce this effect even 

 when the spinal cord has been severed from the 

 brain, for their relation to the grey matter of 

 the cord is such that their state of excitement 

 is readily conveyed to it. 



3. According to a third hypothesis, it is as- 

 sumed that all the spinal and encephalic nerves, 

 of whatever function, are implanted in the 

 grey matter of the segments of the cerebro- 

 spinal centie with which they are severally con- 

 nected, and do not pass beyond them. The 

 several segments of the cerebro-spinal axis are 

 connected with each other through the conti- 

 nuity of the grey matter from one to another, 

 and through the medium of commissural fibres 

 which pass between them. Through these 

 means, motor or sensitive impulses may be 

 propagated from segment to segment ; and a 

 stimulus conveyed to any segment from the 

 periphery may either simultaneously affect the 

 brain and cause a sensation, or it may be re- 

 flected upon the motor nerves of that segment 

 and stimulate their muscles to contract. Or 

 both these effects may take place at the same 

 moment, as a result of one and the same sti- 

 mulus. According to this hypothesis, each 

 segment of the cord, so long as it retains its 

 proper commissural connection with the brain 

 (by commissural fibres and continuous grey 

 matter), is part and parcel of the centre of voli- 

 tion as well as of that of sensation, and the 

 mind is as directly associated with each seg- 

 ment of the cord as it is with any portion of 

 the encephalon. Let that commissural con- 

 nection be dissolved, and the mind will imme- 

 diately lose its hold upon the cord; but the 

 various segments of that organ may nevertheless 

 still be acted upon by physical impulses, and 

 may still continue to evolve the nervous force in 

 connection with the natural changes which may 

 take place within. 



I am not aware that this view of the me- 

 chanism of the various actions of the nervous 

 system had been ever distinctly enunciated 

 before it had been stated by Mr. Bowman and 

 myself in our work on the Physiological Ana- 

 tomy and Physiology of Man, in 1845.* There 

 is nothing, however, in this hypothesis at 

 variance or inconsistent with the views of 



* The Physiological Anatomy and Physiology of 

 Man, by R. B. Todd and VVm. Bowman, vol. i. 

 p. 323. 



Prochaska ; for this physiologist seems to 

 have held the opinion that the nerves are im- 

 planted in the segment of the cerebro-spinal 

 axis into which they sink, and do not pass 

 beyond it. 



I shall now examine into the merits of each 

 of these hypotheses, and, first, of the excito- 

 motory hypothesis. 



It is unnecessary to repeat the objections 

 already stated (p. 721 B) to the use of the 

 term excito-moton/. I shall only remark that 

 some of these objections are equally opposed 

 to the hypothesis as to its name. 



Nevertheless this hypothesis has much to 

 commend it: and not the least argument in its 

 favour is that drawn from the compound nature 

 of spinal nerves, as proved by Bell, in which 

 nerve-fibres of different endowments are bound 

 together in the same sheath. If it be proved 

 (as it has been) that fibres of sensation and of 

 motion may be thus placed in juxtaposition 

 in the same nervous trunk, it seems not an 

 unreasonable conjecture that fibres of other 

 function (excitors and their corresponding mo- 

 torn) might be enclosed in the same sheath 

 with them. 



Both anatomy and experiment, however, unite 

 to prove the existence of sensitive fibres distinct 

 from motor fibres; they are found separate in the 

 roots of the nerves, and combined in the nervous 

 cords : but neither anatomy nor experiment fa- 

 vours the existence of a distinct series of excitor 

 and of corresponding motor nerves. Anatomical 

 research affords not the slightest indication of 

 such a series of nerves. And experiments on 

 the roots of the nerves, where it might reason- 

 ably be expected that the excitors would be 

 separated from the motors (following the ana- 

 logy of the motor and sensitive fibres), are by 

 no means favourable to the existence of such 

 fibres in the roots. The failure of experi- 

 menters to excite motion by irritation of the 

 posterior roots of the spinal nerves has been 

 already referred to. A new and extensive 

 series of experiments is much needed to settle 

 this question. I would remark that galvanism 

 should not be used in them, as the results of 

 stimulation by that agent are extremely fal- 

 lacious, from its liability to extend beyond the 

 parts included between the electrodes. 



Other very serious anatomical objections 

 may be urged to this hypothesis. It supposes 

 the existence of two sets of fibres in the spinal 

 cord. Evidence in favour of these is wanting 

 just as much as in favour of those in the roots 

 of the nerves. Many facts favour the conclu- 

 sion that the fibres which constitute the roots of 

 the nerves of any segment of the cerebro-spinal 

 centre are implanted in the grey matter of that 

 segment, and that none of them are continued 

 beyond that segment up into the brain. They 

 penetrate the spinal cord more or less obliquely, 

 and form their connection with the grey matter 

 a little higher up than the point of penetration ; 

 but there is no evidence to show that they 

 assume a completely vertical direction to pass 

 up to the brain. 



The form and varying dimensions of the 

 spinal cord in its several regions are opposed 

 to this view. If the sensori-volitional fibres are 



