328 FUNCTIONS OF THE NERVOUS SYSTEM. 



in them. Such an idea is further warranted by the history of their develop- 

 ment ; for we find, in the Human embryo of the sixth week, a distinct vesi- 

 cle for the Thalami Optici, interposed between the vesicle of the Corpora 

 Quadrigemina, and that which gives origin to the Cerebral Hemispheres; 

 whilst the Corpora Striata constitute the floor of the cavity or ventricle, which 

 exists in the latter. Now, as already pointed out, we may distinguish in the 

 Medulla Oblongata and Crura Cerebri, a sensory and a motor tract ; by 

 the endowments of the nerves which issue from them. The sensory tract 

 may be traced upwards from the Olivary columns, until it almost entirely 

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

 nerves, and the peduncles of the Olfactive, 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 tracts of the Crura Cerebri, which descend into the Pyramidal co- 

 lumns ; and their connection with the motor function is very generally admit- 

 ted, from the constancy with which paralysis is observed to accompany lesions 

 of these bodies, even when they are affected to a very trifling extent. 



424. The Thalami Optici, and the Corpora Striata, as is well known, are 

 very closely connected with each other by commissural fibres ; and, if the 

 preceding account of their respective offices 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 impulses in respondence to sensations excited through 

 the former. They are also closely connected with other ganglionic masses 

 in their neighbourhood, such as the locus niger, and the vesicular matter of 

 the pons ; which again, are in close relation with the vesicular matter of the 

 medulla oblongata. Altogether it is very evident, that an extensive tract of 

 ganglionic matter exists at the base of the Encephalon, which is really just as 

 distinct from either the Cerebrum or Cerebellum, as these are from each 

 other ; and we have next to inquire, what functions are to be assigned to it. 



425. The determination of these may seem the more difficult, as it is im- 

 possible to make any satisfactory experiments upon the ganglionic centres in 

 question, by isolating them from the Cerebral Hemispheres above, and from the 

 Medulla Oblongata and Spinal Cord below. But the evidence derived from 

 Comparative Anatomy appears to be in this case particularly clear ; and, 

 rightly considered, seems to afford us nearly all the information we require. 

 In the series of " experiments prepared for us by nature," which is presented 

 to us in the descending scale of Animal life, we witness the effects of the 

 gradual change of the relative development of the Sensory ganglia and Cerebral 

 Hemispheres, which are presented to us in the Vertebratcd classes ; and the results 

 of the entire withdrawal of the latter, and of the sole operation of the former, 

 which are presented in the higher Invertebrata. In the sketch already given 

 of the Comparative Anatomy of the Encephalon in Vertebrata, it has been 

 shown that the Sensory ganglia gradually increase, whilst the Cerebral hemi- 

 spheres as regularly diminish, in relative size and importance, as we descend from 

 the higher Mammalia to the lower, from these to Birds, thence to Reptiles, 

 from these, again, to the higher Fishes, in which the aggregate size of the 

 Sensory ganglia equals that of the Cerebrum, thence to the lower Fishes, 

 in which the size of the Cerebral lobes is no greater than that of a single pair 

 of sensory ganglia, the Optic, and frequently even inferior, and lastly, to the 

 ftmphioxus or Lancelot, the lowest Vertebrated animal of which we have 



