The grey matter of the convolutions of the brain presents the same characters throughout, excepting in certain convolutions of the poste rior lobe near the posterior and inferior horns of the lateral ventncles. Ilere, we may ob serve, in a horizontal section, the grey matter of the convolutions sepamted into two portions by a delicate white line, well represented in fig. 398. This layer of light matter was first described by Vicq d'Azyr, but has attracted very little attention from subsequent anatomists.
I have never looked for it without finding it. It consists of nucleated particles, similar to those in the grey matter of the cerebellum. The layer of grey matter external to it contains few nerve-fibres; that internal to it contains them in great numbers, passing into it at right angles.
It is not intended in this part of the article to discuss the physiology of the brain. But in order to develope more clearly than can be done in a mere description the connection of its several parts and the views of its struc ture which I believe to have the best founda tion, I shall state briefly what appears to be the probable modus operundi of the organ, whether as the source of voluntary action or as the re cipient of sensitive impressions.
It will be necessary first to state the follow ing propositions as postulates.
1. The vesicular matter is the source of ner vous power. In mental actions it is the part immediately associated with changes of the mind : whether in the working of the intellect, or in the exercise of the will, or in the per ception of sensitive impressions.
2. The convolutions are the parts immedi ately concerned in the intellectual operation.
3. The simple exercise of the will, for a voluntary movement, is probably connected with the corpora striata.
4. The mere reception of sensitive impres sions is connected with the optic thalami and the superior layer of the crus cerebri.
5. Mental emotions affect the posterior and superior part of the mesocephale.
6. The cerebellum is the regulator of the locomotive actions.
These propositions, which, it is admitted, although not improbable, are far from being proved, svill serve as the basis of an hypothesis of the action of the brain.
In simple operations of thought, as in the exercise of the reasoning powers, or of those of the imagination, the convolutions of the brain are immediately engaged. We do not say. that
material changes give rise to the mental actions, but rather that the changes of the immaterial mind and those of the vesicular matter of the convolutions are simultaneous.
If an intellectual act gives rise to the exer cise of the will, the change in the superficial vesicular matter is propagated by the fibres of the hemisphere to the corpus striatum, whereby the will is excited, and the change in the vesi cular matter of that body is propagated along the inferior layer of the crus cerebri and, after passing through the mesocepliale, along the an terior pyramids to the spinal cord, each ante rior pymmid acting upon that antero-lateral column of the cord which is on the opposite side of the body to itself.
The pyramids connect the vesicular matter of the corpora striata with that of the spinal cord ; from their small size it is highly impro bable that they can be viewed AS continuations of spinal nerves up into the bmin.
Simple solution of continuity of the fibres of the hemispheres, which does not cause pressure, nor affect in any way the corpora. striata, would therefore merely cut off the communication between the seat of intellectual action and the centre of voluntary action. The will, although unaffected, is unable to keep up with the train of thought, and mental con fusion is the result. The loss of speech, which sometimes. precedes a paralytic attack, and which may remain even after the paralysis has been removed, may be accounted for in this way. The intellect is competent to shape the thought, but unable to excite the will, upon which the exercise of the organs of speech is so obviously dependent.
Changes, originating or excited in either hemisphere, may be propagated to the corre sponding parts of the other hemisphere by the transverse commissures, the corpus callosurn, anterior commissure, &c. How far both he mispheres are in simultaneous action, during the rapid changes of the mind in thought, can scarcely be determined ; it seems prolxible, however, that, in certain acts of volition, one only is the seat of the change which prompts to the movement. If I will to move my right arm, the change by which that movement is prompted belongs to the left hemisphere and , corpus suiatum.