And it must be admitted that the intimate connection of sensation and motion, whereby sensation becomes a frequent excitor of mo tion,—and voluntary motion is always, in a state of health, attended with sensation,— vvould a priori lead us to look for the respec tive centres of these two great faculties, not only in juxta-position, but in union at least as 'intimate as that which exists between the corpus striatum and optic thalamus, or between the anterior and the posterior horns of the spinal grey matter.
Saucerotte, Foville, Pinel Grandchamps, and others, advanced the crpinion that the corpora striata and the fibrous substance of the anterior lobes of the brain had a special influence upon the motions of the lower extremities, and that the optic thalami and the fibrous substance of the middle and posterior part of the brain pre sided over the movements of the upper ex tremities. We find, however, but little to favour this theory either in the results of ex periments, in pathological observation, or the anatomy of the parts. Longet states, that, in his experiments upon the optic thalami, the paralysis affected equally the anterior and the posterior extreznities. Andral analysed se venty-five cases of cerebral lesion limited to the corpus striatum or optic thalamus. In twenty-three of these cases, the paralysis was confined to the upper extremity : of these, eleven were affected with lesion of the corpus striatum or of the anterior lobe; ten with lesion of the posterior lobe, or of the optic thalamus; and two with lesion of the middle lobe.* Hence it is plain that a diseased state of the corpus striatuin is as apt to induce pztralysis of the upper extremity as lesion of the tha lamus ; and we are forced to concluile, that pathological anatomy is not competent to de cide the question. Lastly, the anatomy of these two bodies renders it highly improbable that they perform a function -so similar, as that of directing the movements of particular limbs. The great size of the optic thalamus, its mul titude of fibrous radiations, its extensive con nections both in the medulla oblongata and in the hemispheres by means of coinmissural fibres, the marked difference of its structure from that of the corpus striatum, its con nection more with the posterior horns of the spinal grey rnatter than with the anterior ones, and its intimate relation to nerves of sensation, are sufficient anatomical facts to wanant the opinion that the thalami must perform a func tion which, although it may be subservient to, or associated with, that of the striated bodies, is yet entirely dissimilar in kind.
It has been supposed that the corpora striata are special centres or ganglia to the olfactory nerves, and to the sense of smell. But such a supposition is altogether superfluous, inas much as a very distinct and obvious centre to these nerves exists in the olfactory process or lobe, naiscalled nerve by descriptive anatomists. The small olfactory nerves are implanted in the anterior extremity or bulb of this process, which is provided with all the structural cha racters of a nervous centre, and contains a ventricle. This lobe, moreover, is always de
veloped in the diiect ratio of the size and number of the olfactory nerves, and of the developement of the sense of smell ; mid in the Cetacea, a class in which the olfactory nerves and process either do not exist at all, or are so imperfectly developed as to have escaped the notice of some of the ablest ana tomists, the corpom striata are of good size proportionally to that of the entire brain.
Corpora quadrigenaria.—The marked con nection of these gangliform bodies with the optic nerves plainly indicates that they bear some special relation to those nerves, and to the sense of vision ; and this indication be comes more certain when we learn, from com parative anatomy, that in all vertebrate tribes in which the encephalon is developed, special lobes exist, bearing a similar relation to the optic nerves. When the optic nerves are large, these lobes are large; and in the Pleuronecta, in which the eyes are of unequal size, Gottsche states that the optic lobes are unequal, and are related in size to each other, as the eyeballs are. Still, as Serres has remarked, the quadrigeminal tubercles probably perform some other office besides that which refers to vision ; inasmuch as the absence, or extremely diminutive size, of the optic nerves in some animals (the mole for instance) does not materially affect that of these bodies.* Flourens found that destruction of either of these tubercles on one side was followed by loss of sight of the opposite side, and con sequently that the removal of both deprived the animal altogether of the power of vision, but did•not affect its locomotive or intellectual powers, nor its sensibility, except to light. In these experiments the action of the iris was not impaired if the tubercles were only par tially removed ; as long as any portion of the roots of the optic nerves remained uninjured, the iris continued to respond to the stimulus of light, but the total removal of the tubercles paralysed the irides. If the lobes of the brain and cerebellum vvere removed, leaving the tubercles untouched, the irides would continue to contract. These experiments leave no room to doubt that the optic tubercles are the ence phalic recipients of the impressions necessary to vision, which doubtless are simultaneously felt by means of the optic thalami ; and that they are the centres of those movements of the iris which contribute largely not only to pro tect the retina, but likewise to increase the perfection of vision. The optic nerve is at once the nerve of vision, and the excitor of motor impulses which are conveyed to the iris by the third nerve, which takes its origin very near to the optic tubercles. It is interesting to add, that irritation of an optic tubercle on one side causes contraction of both irides :— this is quite in accordance with the well established fact, that, if light be admitted to one eye so as to cause contraction of its pupil, the other pupil will contract at the sarne time. So simultaneous is the action of the two cen tres; so rapid must be the tmnsmission of the stimulus from one side to the other.