722H 
the lumbar portion perfectly whole, the sphincter 
will nevertheless be paralysed. In the former 
instances, the centre of volition in the cranium 
is diseased ; in the latter, the defect consists in 
the destruction of the communication of the 
brain with that portion of the cord in which 
the nerves of the sphincter inuscle are im- 
planted. 
An examination of the action of the sphinc- 
ter will show, as has been already noticed, that 
the anus is kept closed ordinarily by the passive 
contraction of the muscle itself; but that its 
active contractions are mainly excited by vo- 
luntary influence, allowance being made for 
some slight action which may be produced by 
the stimulus of sudden distension, as in other 
circular muscles. Now, as a stimulus to sen- 
tient nerves constitutes no necessary part of 
any of these actions, it is probable that the 
motor nerves of the sphincter have little or no 
connection with the sentient ones; and, conse- 
quently, that muscle is not usvally excitable to 
contraction by a stimulus appled to a sentient 
surface. Hence, whenever the influence of the 
will upon the lumbar portion of the cord is 
suspended, this muscle ceases to act, whether a 
mental or a physical stimulus be exerted. 
We have remarked before that all that is 
shown by Dr. Hall’s experiments on the horse 
and on the turtle is that the spinal cord influ- 
enced the sphincter only whilst it was in a state of 
irritation consequent upon its division. There 
probably was no real reflex action at all, and the 
closure of the anus on the application ofa stimu- 
lus was probably only apparently due to that 
cause, frequent contractions taking place in the 
muscle in effect ofthe irritated state ofthe cord. 
On the same principle, animals will exhibit 
movements of voluntary character for some 
time after decapitation, the continued irritation 
of the cord acting asa stimulus. A bird thus 
treated will fly for some distance, and with 
considerable energy, and will flap its wings if 
the cut surface of the cord be irritated. A fly 
decapitated pursues its course for some way 
immediately after the removal of the head; and 
Walckenaer observed a singular fact respecting 
the Cerceris ornata, a wasp which attacks a bee 
that inhabits holes: “ at the moment that the in- 
sect was forcing its way into the hole of the bee, 
Walckenaer decapitated it; notwithstanding 
which, it continued its motions, and, when 
turned round, endeavoured to resume its posi- 
tion and enter the hole.”* The change in the 
vesicular matter of the ganglia necessary for 
the movements of the wasp in pursuit of its 
_prey, had already been excited by a powerful 
stimulus of volition, which continued even 
after the removal of the centre from which it 
had emanated. Actions at first voluntary, 
which by frequent repetition become habitual 
and involuntary, are, no doubt, to be accounted 
for by the persistence of that condition of the 
vesicular matter which the will at first induced, 
and to which the frequency of repetition gives 
a character of permanence. Thus Habit is due, 
* Quoted in Miiller’s Physiol by Baly, vol. i. 
p- Ter 2nd ed. si a a 
PHYSIOLOGY OF THE NERVOUS SYSTEM. 
as it were, to the fixation of a certain state of 
vesicular matter—it is the conversion of a men- 
tal into a physical nervous action by frequent 
repetition. - 
So similar is the change which a physical — 
stimulus can excite in the grey matter to that 
produced by the influence of the will, 
as has been often remarked, the actions exci 
in decapitated animals present a striking re- 
semblance to the ordinary voluntary move- 
ments. When a certain portion of the skin is — 
irritated, the animal pushes against the offend-— 
ing substance, as if trying to remove or dis- 
place it. If the anus be irritated, both 
are excited to action. It may also be o 
that the same motions follow the same irrita- — 
tions of the skin. If, in a frog, the seat of 
irritation be on the right side, the correspond-— 
ing hind-foot will be raised, as if to remove 
the irritating cause. The exact resemblance 
of these to voluntary movements seems to 
admit of being explained only on the suppo: 
sition that the same fibres are employed in he 
execution of both. . 
It must be kept in view, that, while thi 
hypothesis rejects the class of sensori-volition 
fibres which are supposed to pass with the 
spinal nerves along the cord into the brain, it 
admits the existence of only three orders of 
fibres implanted in the various segments of the 
cord, viz. those at once sensitive and excitor, 
those at once for voluntary and involuntal 
motion ; and commissural fibres ; of which he 
former only contribute toform the nerves. J 
must not be supposed, however, that it is inte 
ed by this hypothesis to assume that the inter 
vention of sensation (i.e. the perception of at 
impression by the mind) is necessary for th 
production of those muscular actions 
are excited by stimulation of the surface. . 
more is affirmed than that the same stimuh 
to the sensitive nerve which can and do 
excite a sensation, may simultaneously, bi 
independently, cause a change in the vesie 
matter which shall stimulate the motor nerves, 
and that this change is of the same kind 
that which the will may excite, and affee 
same motor nerves. 
Lastly, this hypothesis involves the € 
ciation of a highly important proposition \ 
reference to nervous centres. It is this: 
all the centres which are connected to the b 
by commissural fibres, are thereby submit 
to, and brought into connection with, the mii 
to an extent proportionate to the numbet 
connecting fibres, so that veluninas imp 
act upon them as part and parcel of the cel 
of volition; and sensitive impressions, 
fecting them, affect the mind simulte 
In voluntary actions, then, it may be st 
that, while the brain is the part primarii 
fected, the mental impulse is also di 
that portion of the cord upon which the 
action depends. oe 
In the developement of sensation the stim 
lus affects the posterior horns of the grey mi 
of the cord, which, from its commiss' : 
nection with the brain, is in reality a part 
the sensorium. When the power of meg 
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