Whether the fibres which Henle has desig nated gelatinous fibres, which resemble very much the central band of the nerve-tubes de prived of tubular membrane and white sub stance, perform a siriailar office, or whether they serve to establish a connection between the grey matter of the several nervous centres, are ques tions which we must leave for future considera tion.
The sagacity of Galen long ago pointed out that every part, which is capable of motion, and at the same time possesses sensibility, must receive two classes of nerves, rnotor and sensitive. And it was reserved for the genius of Bell in our own times to demonstrate that the office of a nerve depends upon the powers or endowments of its component fibres or tubules, and that a nervous trunk may be made up of fibres of different endowments lying in juxta-position with each other.
It is at the roots of the nerves that tubules of distinct endowments are isolated from each other. Thus Bell's experiments, which have been confirmed by subsequent observations, shewed that the anterior roots of spinal nerves were motor, and the posterior sensitive ; and the determination of this important fact is the foundation of all our knowledge of the phy siology of nerves.
The difference in the powers or endowments of the nerve-tubes does not appear to depend upon any variety in their structure, or other physical diameters, (size perhaps excepted,) for repeated examination has failed to detect any such, but rather upon their peripheral and cen tral connexions. A sensitive nerve, while it is
organized at its periphery in such a mariner as to adapt it to the reception of impressions, must be connected with that part of the brain whose office it is to perceive the changes which such impressions can produce. And a motor nerve must be on the one hand connected with mus cular fibres, and on the other associated with such a part of the brain or other nervo 'Li,. centre as is capable of exciting in it that change which when communicated to a muscle will stimulate it to contract.
The precise mechanism of those nervous acts, which I would distinguish as purely physical, by reason of their independence of the mind, is as yet unknown. It is still undetermined whether a distinct series of fibres (excitomotory) is necessary for them, or whether they may not be performed by the same fibres which are the channels of the mandates of the will, and of the impressions of those stimuli which are capable of producing sensation.
(R. B. Todd.)