PHYSIOLOGY OF THE NERVOUS SYSTEM. 
channels for voluntary, emotional, or strictly 
physical impulses to motion. 
The mechanism of a voluntary action in 
parts supplied by spinal nerves would be, 
according to this hypothesis, as follows:— 
The impulse of volition, excited primarily in 
the brain, acts at the same time upon the grey 
matter of the cord (its anterior horn), and 
through it upon the anterior roots of the nerves 
implanted in it. This grey matter, in virtue of 
its association with the brain by means of the 
anterior pyramids, becomes part and parcel of 
the organ of the will, and therefore as distinctly 
amenable to acts of the mind as that portion 
which is contained within the cranium. If we 
destroy the commissural connection with the 
brain through the pyramidal fibres, the spinal 
cord ceases to take part in mental nervous ac- 
tions; or, if that connection be only partially de- 
stroyed, that portion of thecord which the injured 
fibres had associated with the brain is no longer 
influenced by the mind. Again, if the seat of 
volition in the brain be diseased, the cord or part 
of it participates in the effects of the diseaseas far 
as regards voluntary actions. That it is not too 
much to ascribe such power to the pyramidal 
fibres appears reasonable, if we consider how 
the fibres of the corpus callosum, and perhaps 
other transverse commissures, so connect the 
hemispheres and other parts of the brain that 
the separate divisions of a double organ act 
harmoniously so as to excite but a single train 
of thought, or, conversely, that two impressions 
from one and the same source on a double 
sentient organ are perceived as single by the 
mind. 
An objection to this explanation will readily 
be raised—namely, that the excitation of the 
anterior horn of the grey matter, in the way 
Stated, does not explain the remarkable power 
which the will has of limiting its action to one 
or two, or a particular class of muscles. To 
this, however, it may be replied that there can 
be no reason for denying to the mind the 
faculty of concentrating its action upon a par- 
ticular series of the elementary parts of the 
vesicular matter, or even upon one or more 
vesicles, if we admit that it can direct its 
influence to one or more individual fibres, as 
the advocates of the first and second hypotheses 
do. If, indeed, we admit the one, we must 
admit the other; for whether the primary exci- 
tation of a fibre take place in the encephalon 
or in the spinal cord, the part first affected 
must probably be one or more vesicles of grey 
matter. 
The series of changes which would develope 
a sensation admits of the following explana- 
tion according to this hypothesis :—A stimulus 
applied to some part of the trunk or extremities 
18 propagated by the sensitive nerves to the 
posterior horn of the grey matter of the spinal 
cord, and from the junction of this part with 
the brain, either through the direct continuity 
of the vesicular matter of the cord with that of 
the centre of sensation, by the olivary column, 
or through longitudinal commissural fibres, 
analogous to or even forming a part of the an- 
terior pyramids, this is simultaneously affected. 
722E 
To this, likewise, it will be objected that the 
limitation of sensation is not sufficiently ex- 
plained. But the reply is obvious ; the intensity 
and kind of sensation depend upon the nature 
of the primary stimulus at the surface, the 
extent upon the number of fibres there stimu- 
lated. Wherever these fibres form their proper 
organic connection with the vesicular matter, 
that matter will participate in their change to 
an extent proportionate to the number of fibres 
stimulated, and with an intensity commensurate 
with the force of the primary stimulus. It is 
not necessary to the developement of sensation 
that the fibre stimulated should be implanted 
directly in the brain; if it be conrfected with 
this centre through the medium of vesicular 
matter or through commissural fibres, all 
the conditions necessary for the developement 
and propagation of nervous force would appear 
to be fulfilled. It must not be supposed, 
however, that in making this statement we 
mean to assign the spinal cord to be the seat of 
sensation ; all we assert is, that the posterior 
horn of the grey matter, as being the part in 
which the sensitive roots are implanted, is the 
seat of physical change excited by the stimulus 
applied to the sensitive fibres, which change 
must be perceived by the mind before true 
sensation can be produced. In fine, by the 
union of the posterior horns of the spinal grey 
matter with the vesicular matter of the brain, 
they become a part of the centre of sensation 
so long as that union is unimpaired.* 
This hypothesis offers an explanation of the 
hitherto unexplained phenomenon of impaired 
sensation on that side of the body which is 
opposite to the seat of cerebral lesion. If we 
regard the anterior pyramids as commissures 
between the sensitive as well as between the 
motor portions of the cerebro-spinal centre, it 
will be obvious that the posterior horns of the 
spinal grey matter on the right side will be 
associated with the left centre of sensation, and 
vice versa. ° 
And we gain, moreover, an explanation of 
the almost universal association of sensation 
with reflex or physical nervous actions. The 
excitor nerves of these actions being the same 
as the sensitive nerves, the impression con- 
veyed by them is calculated at once to excite 
motion and sensation. The controlling influ- 
ence of the will prevents many of the sensitive 
impressions made through the spinal cord from 
developing corresponding movements. And 
this controlling influence is best explained by 
this hypothesis, for as it admits no other motor 
nerves connected with the cord but those over 
which the will can exert an influence, it follows 
that such mental influence, if more powerful 
than the physical stimulus which the sentient 
nerves convey, may prevail over it and neu- 
tralize its force. On the other hand, under 
certain conditions of great physical excitation, 
(exalted polarity,) physical changes overcome 
* Tn all discussions relative to sensation it should 
be kept in view that true sensation involves a men- 
tal act, namely, the perception of a physical impres- 
sion, and of the concomitant physical change in 
the nervous matter. 
