Certain cases of disease confined to one he misphere, in which a considerable degree, at least, of intellectual power persists, denote that the sound one may suffice for the manifes tation of the changes connected with thought, and it may be reasonably supposed that the sound hemisphere may excite to action the centre of volition (corpus striatuna) on the dis eased side.
The existence of hemiplegic paralysis, then, implies an affection, direct or indirect, of the centre of volition (corpus striatum) on the op posite side. Pressure, or a morbid change in the physical state of its tissue originating in it or propagated to it, is all that is necessary for this purpose; and this change, like the change in the normal actions, may be of such a kind as to elude our means of observation.
When a sensation is excitecl, the stimulus acts from periphery to centre. The change is propagated by the sentient nerve to the optic thalamus, which, by its numberless radiations and its many comrnissures, is well calculated to excite all parts of either hemisphere, and even of both hemispheres. When the nerve excited is one of pure sense, the change is wrought more directly in the bmin; if the fifth, or any of the nerves of the medulla oblongata, the stimulus acts directly on the part ; but if a nerve of either limb be stimulated, the change must be propagated through the spinal cord.
It will be asked, if this be the ntodus ope randi in sensations, how does it happen that disease of one optic thalamus does not impair sensation in one-half of the body? And how is it that such disease is much more frequently associated with hemiplegic paralysis, of a kind not to be distinguished from that which de pends on diseased corpus striatum. The answer to the first question is as follows. The optic thalamus, or, more properly, the centre of sen sations, is never wholly diseased, for this centre is not confined to the optic thalamus of descrip tive anatomists, but extends to the mesocephale and olivary columns. Extensive disease of this centre would probably be fatal to sensa tion. But the most ample provision exists for opening up new channels of sensation if those on one side or a part of them be impeded. The centres of opposite sides are intimately connected, especially in the medulla oblongata and mesocephale, by commissural or by decus sating fibres ; the optic thalami of opposite sides are connected to each other by the poste rior commissure and the soft commissure, and the immense multitude of fibres which radiate from each thalamus insure its connection with a considerable extent of the brain, so that a change in any part of it cannot fail to be com ' municated to some portion of the hemisphere.
I It is sufficient for mere sensation that the centre of sensibility should be affected. Intellectual change resulting from that affection depends illupon fibres which radiate between it and the optic thalami.
It often happens that at the onset of a cerebral lesion sensation as well as motion is paralysed in the opposite side of the body. In a few days, however, the sensibility returns whilst the pa ralysis of motion remains,—a fact which is sufficient to show that the motor and sensitive power must have different channels in . the centres as well as in the nerves. The primary paralysis of sensation may be due to a lesion . on one side affecting the centre of sensibility, or to the shock which that centre may 'have received from the sudden occurrence of lesion in some other neighbouring part. In the latter instance the recovery of sensibility takes place evidently on the subsidence of the effects of shock : in the former it may depend on the existence of other channels of sensitive impres sions, independently of those involved in the lesion. Hence there may be lesion of one optic thalamus without loss of sensibility. The answer to the second question is ob tained from considering the intimate connection of the corpus striatum and optic thalamus. No two parts of the bmin are so closely united by fibres in vast numbers passing from one to the other. Disease of the thalamus therefore may excite a morbid state of the corpus striaturn, without producing any change in its structure, which may be recognised by the or dinary means of observation. And thus he miplegia will take place, and remain as long as the morbid state of the corpus striatum re mains. A lesion of the corpus striatum may in a similar manner affect the optic thalamus of the same side; but as that is not the only channel of sensitive impressions, a loss of sen sibility does not necessarily occur.