510 OF THE NERVOUS SYSTEM AND ITS ACTIONS. 



Sensory Column that ascends from the Posterior Pyramids, we shall 

 find it to enter the Thalami Optici. These bodies have been usually 

 considered as mere appendages to the Cerebrum ; but the fact that they 

 are independent centres of action is fully established by the presence of 

 a large quantity of vesicular matter in their substance ; and there is 

 now a sufficiently large amount of evidence, both anatomical and physio- 

 logical, to render it probable that the fibres which seem to pass through 

 them from the Crura Cerebri, and then to radiate towards the periphery 

 of the Cerebral Hemispheres, do not do so in reality, but that these 

 ganglionic masses receive, on the one hand, the fibres that ascend to 

 them from the Medulla Oblongata, and, on the other, are the point of 

 departure of a new set, passing to the proper Cerebrum. Looking to 

 the connexion of the Thalami Optici with the sensory tract, it may be 

 regarded as not improbable that we may consider them as the ganglionic 

 centres of common sensation ; standing in the same relation to the sen- 

 sory nerves, that converge from various parts of the body towards the 

 Encephalon, as do the Optic and other ganglia to their nerves of special 

 sensation. And as these last give origin to motor fibres, so may we 

 regard the ganglionic matter of the Corpora Striata (which are in close 

 connexion with the Thalami) as probably sharing in the same function ; 

 giving origin to the motor fibres, which produce the respondent con- 

 sensual movements ; just as the anterior peak of gray matter in the 

 Spinal Cord gives exit to the motor filaments, which effect the reflex 

 movements excited through the afferent fibres forming part of the pos- 

 terior roots. 



902. The functions of this series of ganglia may be more certainly 

 determined by the aid of Comparative Anatomy, than by experimental 

 mutilations. Reverting to the class of Fishes, we find that it there 

 constitutes, with the Cerebellum, nearly the entire Encephalon ; scarcely 

 a rudiment of the true Cerebrum being discoverable in that group.* 

 And when we descend to the Invertebrata, we find the Cephalic masses 

 entirely to consist of the ganglionic centres of the nerves of sense and . 

 motion. There can scarcely be a reasonable doubt, that these Cephalic 

 ganglia are the seat of consciousness and the sources of those movements 

 which are directed by sensation, in such animals as present this low 

 type of nervous organization ; and there is no adequate reason for the 

 belief that the superaddition of the Cerebral Hemispheres in the Verte- 

 brated series alters the endowments of the. Sensory Ganglia on which 

 they are superimposed ; on the contrary, we everywhere see that the 

 addition of new ganglionic centres, as instruments of new functions, 

 leaves those which were previously existing in the discharge of their 

 original ^duties. Hence we should be led to regard them as the centres 

 of consciousness, even in Man, each pair of ganglionic centres minister- 

 ing to that peculiar kind of sensation for which its nerves and the organs 

 they supply are ^set apart ; thus we should consider the Optic ganglia to 

 be the seat of Visual sensations, the Auditory to be the seat of the sense 

 of hearing, and so on. And we should also consider them as the instru- 



* The ganglionic masses, commonly designated as the Cerebral lobes or hemispheres, 

 must be really likened in great part (as already stated 869) to the Corpora Striata. 



