So similar is the change which a physic-al stimulus can excite in the grey matter to that produced by the influence of the will, that, as has been often remarked, the actions excited 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 legs are excited to action. It may also be observed, that the same motions follow the same irrita tions of the skin. lf, 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 the execution of both.
It must be kept in view, that, while this hypothesis rejects the class of sensori-volitional 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 involuntary motion ; and commissural fibres ; of which the former only contribute to form the nerves. It must not be supposed, however, that it is intend ed by this hypothesis to assume that the inter vention of sensation (i.e. the perception of an impression by the mind) is necessary for the production of those muscular actions which are excited by stimulation of the surface. No more is affirnied than that the same stimulus to the sensitive nerve which can and does excite a sensation, may simultaneously, but indipcndently, cause a change in the vesicular matter which shall stimulate the motor nerves; and that this change is of the same kind as that which the will inay excite, and affects the same motor nerves.
Lastly, this hypothesis involves the enun ciation of a highly important proposition with reference to nervous centres. It is this : that all the centres which are connected to the limn by commissural fibres, are thereby submitted to, and brought into connection with, the niind, to an extent proportionate to the number of connecting fibres, so that voluntary impulses act upon them as part and parcel of the centre of volition ; and sensitive irnpressions, in af fecting them, affect the mind simultaneously.
In voluntary actions, then, it may be stated that, while the brain is the part primarily af fected, the mental impulse is also directed to I that portion of the cord upon which the required action depends.
In the developement of sensation the stimu lus affects the posterior horns of the grey matter of the cord, which, from its comrnissural con nection with the brain, is in reality a part of the sensorium. When the power of mental interference is removed, or kept under con trol, physical actions develope themselves ; being eflected through the same nerves as those which volition influences or which sensitive impressions affect. The latter are, in such instances, the excitors of the former, no doubt through the vesicular matter in which they are implanted. These actions become most mani fest when the connection of the brain with the spinal cord has been severed ; and they occur in the most marked way in those situations where the cutaneous nerves are so organized as readily to respond to the application of a stimulus applied to the surface, or they be come universal when the cord is in a state of general excitement.
The movements in locomotion and the main tenance of the various attitudes are effected through the ordinary channels of the physical and volitional actions ; and the posterior co lumns of the cord, by their influence on the vesicular matter of the segments in which the nerves are implanted, co-ordinate and har monize the complicated muscular actions of the limbs and the trunk under the controul of that portion of the encephalon which probably is devoted to that purpose. This power of co ordination is probably mental, and -intimately connected with the muscular sense.