650 FUNCTIONS OF THE CEREBRO -SPIN AL NERVOUS SYSTEM. 



longitudinal nerve-fibres resting O u an arched transparent lamina, which 

 forms the roof of the Sylvian Aqueduct. A large proportion of the Optic 

 tract is connected with" the nates. But besides these, at the base of the 

 Cerebral Hemispheres, we find two other large gaugliouic masses on either 

 side, through which nearly all the fibres appear to pass that connect the 

 Hemispheres with the Medulla Oblongata: namely, the Thalami Optici and 

 the Corpora Striata. Now, although these are commonly regarded in the 

 light of appendages merely to the Cerebral Hemispheres, it is evident from 

 the large quantity of vesicular matter they contain, that they must rank as 

 independent ganglionic centres; and this view is supported alike by the 

 evidence of Comparative Anatomy, and by that afforded by the history of 

 Development. For it is certain that the size of the Thalami Optici and 

 Corpora Striata presents no more relation, in different tribes of animals, to 

 that of the Cerebrum, than does that of the ganglia of Special Sense ; and 

 they may even present a considerable development, when the condition of 

 the Cerebrum is quite rudimentary. Thus, in the Osseous Fishes, a careful 

 examination of the relations of the body which is known as the Optic lobe 

 (Fig. 193, c) makes it apparent that this is the representative, not merely of 

 the & proper Optic ganglion of Man, but also of the Thalamns Opticus ; whilst, 

 again, the mass which is designated as the Cerebral lobe (B) is chiefly homol- 

 ogous with the Corpus Striatum of higher animals. The nature of the latter 

 body is made apparent, in the higher Cartilaginous Fishes, by the presence 

 of a ventricle in its interior ; the floor of this cavity being formed by the 

 Corpus Striatum, whilst the thin layer of nervous matter which forms its 

 roof is the only representative of the Cerebral hemisphere. So in the human 

 embryo of the sixth week, we find a distinct vesicle for the Thalami Optici, 

 interposed between the vesicle of the Corpora Quadrigemiua and that which 

 gives origin to the Cerebral Hemispheres ; whilst the Corpora Striata consti- 

 tute the floor of the cavity or ventricle which exists in the latter, this being 

 as yet of comparatively small dimensions. Now, as already pointed out 

 ( 488), we may distinguish in the Medulla Oblongata and Crura Cerebri, 

 a sensory and amotor tract, by the endowments of the nerves which issue from 

 them. v lhe sensory tract may be traced upwards, until it almost entirely 

 spreads itself through the substance of the Thalamus. Moreover, the Optic 

 nerves and the peduncles of the Olfactory may be shown to have a distinct 

 connection with the Thalami, the former by the direct passage of a portion 

 of their roots into these ganglia, and the latter through the medium of the 

 Fornix. Hence we may fairly regard the Thalami Optici as the chief focus 

 of the Sensory nerves, and more especially as the ganglionic centre of the 

 nerves of common sensation, which ascend to it from the Medulla Oblongata 

 and Spinal Cord. -On the other-hand, the Corpora Striata are implanted on 

 the Motor tract of the Crura Cerebri, which descend into the Pyramidal 

 columns; and their relation to the fibres of which that tract is composed, 

 appears to be essentially the same as that which the Thalami bear to the 

 sensory tract. The Corpora Striata are connected with each other, on the 

 median plane, by the. anterior commissure; and the Thalami Optici, by the soft 

 and the. interior commissures. The Corpus Striatum and Thalamus Opticus 

 of the same side are very closely connected by commissural fibres, st retching 

 from one to the other ; and, if the preceding account of the respective offices 

 of these bodies be correct, they may be regarded as having much the same 

 relation to each other as that which exists between the posterior and anterior 

 peaks of vesicular matter in the Spinal Cord ;' the latter issuing motor im- 



i This was first pointed out by Messrs. Todd and Howmun, in their Physiological 

 Anatomy, vol. i, pp. 347-350. 



