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 thmugh 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 the cord 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 disease as 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 callosurn, 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 org,an 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 is 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 tenor pyramids, this is simultaneously affected.
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 mediurn 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 versh.
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 mental stimuli, and the mind loses all control; this is the case in poisoning by strychnine, in tetanus, in convulsions.