722B 
2. A second hypothesis is that which accords 
with the views of Miiller 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 centre 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 
poe may either simultaneously affect the 
rain 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, asa 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 Wm. Bowman, vol. i. 
p. 323. 
PHYSIOLOGY OF TIE NERVOUS SYSTEM. 
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. 7218) to the use of the — 
term excito-motory. 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- 
tors) might be enclosed in the came shell 
with them. . 
Both anatomy and experiment, however, unite 
to prove the existence of sensitive fibres distinct 
from motor fibres; they are found separate in t 
roots of the nerves, and combined in the 
cords: but neither anatomy nor experiment fa-— 
vours the existence of a distinct series of excite 
and of corresponding motor nerves. Anatomical 
research affords not the slightest indication of 
such a series of nerves. And experiments ¢ 
the roots of the nerves, where it might reason- 
ably be expected that the excitors would b 
separated from the motors (following the ana 
logy of the motor and sensitive fibres), are by 
no means favourable to the existence of sue 
fibres in the roots. The failure of experi- 
menters to excite motion by irritation of th 
posterior roots of the spinal nerves has been 
already referred to. A new and extensiv 
series of experiments is much needed to 
this question. I would remark that galvani 
should not be used in them, as the results 
stimulation by that agent are extremely 
lacious, from its liability to extend beyond 
parts included between the electrodes. 
Other very serious anatomical objects 
may be urged to this hypothesis. It suppo 
the existence of two sets of fibres in the spit 
cord. Evidence in favour of these is war 
just as much as in favour of those in the rot 
of the nerves. Many facts favour the conel 
sion that the fibres which constitute the root 
the nerves of any segment of the cerebro-sp 
centre are implanted in the grey matter of 
segment, and that none of them are contint 
beyond that segment up into the brain. — 
penetrate the spinal cord more or less obliqa 
and form their connection with the grey m 
a little higher up than the point of penetratio 
but there is no evidence to show that the) 
assume a completely vertical direction to pas 
up to the brain. a 
The form and varying dimensions of 
spinal cord in its several regions are oppost 
to this view. If the sensori-volitional fibr 
wow 
