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mental stimuli, and the mind loses all control ; 
this is the case in poisoning by strychnine, in 
tetanus, in convulsions. 
The difference of structure of the anterior 
and posterior horns of the vesicular matter of 
the spinal cord may be appropriately referred 
to as indicating a difference in functions be- 
tween these horns. The anterior horns contain 
large caudate vesicles of a remarkable and 
peculiar kind, containing a considerable quan- 
tity of pigmentary matter; the posterior horns 
resemble very much in structure the vesicular 
matter of the cerebral convolutions and of 
other parts of the cerebrum, and do not contain 
caudate vesicles, except near the base. Here, 
then, we find associated with the well-attested 
difference in the functions of the anterior and 
posterior roots, a striking difference in the 
structure of the anterior and posterior horns of 
the spinal grey matter in which they are re- 
spectively implanted. 
We gain from this hypothesis that which 
neither of the others could supply, namely, an 
explanation of the influence of emotion on 
limbs paralysed to volition. Mental emotion 
excites a change in the brain, probably in that 
part which forms the upper and posterior por- 
tion of the mesocephale: this change is readil 
propagated to the spinal grey matter thrown 
the olivary columns, independently of the py- 
ramidal fibres. The spinal grey matter being 
excited, the nerves implanted in it are stimu- 
lated, and motions are produced closely resem- 
bling those which the will can develope. 
We have noticed that the will can control 
reflex or other physical nervous actions. When 
the influence of the will is- suspended, reflex 
actions may be more easily excited. These 
facts admit of the most obvious explanation by 
the hypothesis under examination. 
Some reflex actions are imperfectly control- 
lable by the will; such as the contraction of 
the pupil, and the movement of deglutition at 
the isthmus faucium. This, however, cannot 
be cited as at all opposed to the view we are 
advocating ; for there is nothing in this hypo- 
thesis repugnant to the idea that certain nerves 
may be connected in the nervous centres with 
masses of vesicular matter over which the will 
usually exercises little or no control, and which, 
perhaps, may have but a slight connection 
with the centre of volition through commissural 
fibres. Still, respecting the two actions above- 
mentioned, it must be remarked that in de- 
glutition the mental influence is not sufficient 
by itself: we cannot perfectly contract the fau- 
ces, if food or some other physical stimulus be 
not present; the double stimulus—physical, as 
of the food, and mental, the will—appears 
necessary for the perfect performance of this act. 
In theaction of the pupil, the mental stimulus 
can only be brought to bear on the pupil, by di- 
recting it to another muscle at the same time, 
namely, the internal rectus muscle of the eyeball. 
When the eyeball is directed toward the nose, 
the pupil is usually simultaneously contracted. 
A double stimulus, mental and physical, 
appears to be necessary to the perfect develope- 
ment of many actions. This by potuesis offers 
PHYSIOLOGY OF THE NERVOUS SYSTEM. 
a ready explanation of the way in which the — 
two stimuli may combine to promote the same 
action. The:mental stimulus acts directly on 
the vesicular matter, the physical is propagated 
to it by sensitive nerves; and thus both acting 
on the same region of vesicular matter excite 
the same motor nerves. We have already no- 
ticed how this takes place in deglutition at the 
isthmus faucium. In locomotion there can be — 
no doubt that the double stimulus is in opera- 
tion: the degree of contraction of the muscles 
of the lower extremities necessary to maintain 
the superincumbent weight is obtained by the 
physical stimulus of pressure against the soles 
of the feet, where the skin is peostiets fitted 
for the reception of such a stimulus; but the 
movements of the limbs, and the harmonizi 
association of the muscular actions, are eflechall 
by mental influence. The pressure against the 
soles is felt, however, and the skin of the soles 
is known to be highly sensitive ; and the same — 
nerve-fibres which excite the sensation stimulate 
the vesicular matter in which the motor nervesare 
implanted. In many actions of familiar oe 
currence the voluntary effort is greatly en h 
by the simultaneous application of a physical 
stimulus to a part of the surface which is sup- 
plied with nerves from the same region of the 
cord. The horseman feels more secure when 
his legs are in close contact with the horse's 
flank. We gain a much firmer hold of an 
object which adapts itself well to the palmar 
surface of the hand, than of one which, al- 
though of no greater bulk, is yet so irregular 
in surface as not to allow of such intimate con- 
tact with the palm. Closure of the eyelids ig 
winking is an action of similar kind, resulting 
from a physical stimulus, which in the pe 
state of the cerebro-spinal centre produces se 
sation, and excites motion which is at once th 
result of the physical impression, and of th 
exercise of volition provoked by the sensation 
Every one must be conscious that he exere 
considerable control over the movements of h 
eyelids, and that it requires a great effort i 
revent winking for a certain period. | 
ength, however, the physical impression, @ 
ing from the contact of air with the conjuneti 
and the diminution of temperature from @ 
poration on the surface of that memb 
which at first caused but a slight sensat 
produces pain; the physical stimulus ¢ 
comes the mental resistance, and causes & 
traction of the orbicular muscle. And it 
be remarked further, that the closure ¢ 7 
lids by voluntary effort is much more powel 
if a stimulus be applied at the same tif 
the conjunctival surface, than if left so 
the exercise of the will. 
In the action just referred to, as well 
all other instances of reflex actions 
will can prevent, no satisfactory éxplan 
this controlling power of the mind can be gi 
by Dr. Hall’s hypothesis. Do the volit 
fibres exceed in number the excito-motory! 
this were admitted, then we could understat 
that an excito-motory act might be p 
by substituting a voluntary act for it; but 
the cases in question, the mind prevents ac 
Z Me 
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