Optic thalanti.—The same line of argument which teads us to view the corpora striata as the more essential parts of the nervous appa ratus which controul direct voluntary move ments, suggests that the optic thalami may be viewed as the principal foci of sensibility, without which the mind could not perceive the physical change resulting from a sensitive im pression.
The principal anatomical fact which favours this conclusion is the connection of all the nerves of pure sense, more or less directly, with the optic thalami or with the olivary co lumns. The olfactory processes, which appa rently have no connection with them, form, no doubt, through the fomix, such an union with them, as readily to bring them within the in fluence of the olfactory nerves.
According to this sense of its office we must regard the optic thalami as the upper and chief portions of an extended centre, of which die lower part is formed by the olivary columns, which we have already referred to as taking part in the mechanism of sensation. The con tinuity of the olivary columns with the optic thalami justifies this view : nor is it invalidated by the fact, that some of the nerves which arise from the medulla oblong,ata are motor in function ; for Stilling's researches render it probable that these fibres have their origin in special accumulations of vesicular matter, which contain caudate vesicles of the same kind as those ffiund in the anterior horns of the grey matter of the cord.
The results which experiments have yielded add little that is positive to our knowledge of the functions of these bodies. Flourens found that neither pricking, nor cutting away the optic thalami by successive slices occasioned any muscular agitation, nor did it even induce con traction of the pupils. Longet found that re moval of one optic thalamus in the mbbit was followed by paralysis on the opposite side of the body. It appears, however, that this was done after the removal of the hemisphere and corpus striatum, whereby the experiment was so complicated as to invalidate any conclusion that might be drawn from it respecting the function of the thalamus. Indeed, vivisections upon so complex an organ as the brain are ill calculated to lead to useful or satisfactory results; •but one does not hesitate to refer to such as have been made, because they afford a certain amount of negative information, imper fect though it be.
Nothing definitive respecting the proper office of the thalami can be obtained from pa thological anatomy. Extensive disease of these bodies is attended with the same phe nomena during life, as lesion of similar kind in the corpora striata. Hemiplegic paralysis accompanies both; nor does it appear that sen sation is more impaired when the thalamus is diseased, than when the corpus striatum is affected.
There is nothing in the phenomena attendant on morbid states of the thalami which tan be fairly regarded as opposed to the conclusion which their anatomical relations indicate, name ly, that they form a principal part of the centre of sensation. The intimate connection between the striated bodies and the thalami sufficiently explains the paralysis of motion which follows disease of the latter ; whilst, as the thalami do not constitute the whole centre of sensation, but only a part thereof, it cannot be expected that lesion of this part would destroy sensation, so long as the remainder of the centre on the same side, as 'well as that of the opposite side, retain their integrity. Complete paralysis of sensation on one side is very rare in diseased brain : a slight impairment of it frequently exists in the early periods of cerebral lesion, apparently as an effect of shock ; for it quickly subsides, although the motor power rnay never return.
According to the views above expressed, the corpora striata and optic thalami bear to each other a relation analogous to that of the ante rior to the posteiior horn of the spinal grey matter. The corpora striata and anterior horns are centres of motion; the optic thalami and posterior horns, centres of sensation. The anterior pyramids connect the former; the olivary columns, and perhaps some fibres of the anterior pyramids, the latter. The olivary columns, however, are in great part continu ations of the thalami on the one hand, and of the grey matter of the cord on the other; and contain abundance of vesicular matter, in which nerves are implanted.