CEREBRO-SPINAL NERVOUS CENTRES IN GENERAL. 



567 



193. 



..A 



,7-B 



...C 



Brain of Cod : A, olfac- 

 tive ganglia; B, cerebral 

 lobes; c, optic ganglia; D, 

 cerebellum. 



bral Hemispheres (Fig. 193. B) being usually inferior iu size to the Optic 

 ganglia (c) alone. Indeed, of the pair of ganglionic 

 masses to which that designation is usually applied, it 

 may he almost positively stated, that the greater part 

 is homologous with the Corpora Striata of the Human 

 Brain; it being only in the higher Cartilaginous fishes, 

 that a ventricular cavity exists in each of these bodies, 

 separating the thin layer of true Cerebral substance 

 which overlies it, from the gangliouic mass which forms 

 its floor. Between these two extremes, a regular gra- 

 dation is presented in the intermediate tribes. Now it 

 is a point especially worthy of note, that no sensory 

 uerves terminate directly in the Cerebrum, nor do any 

 motor nerves issue from it ; and there seems a strong 

 probability that there is not (as formerly supposed) a 

 direct continuity between all or even any of the nerve- 

 fibres distributed to the body, and the Medullary sub- 

 stance of the Cerebrum. For whilst the nerves of 

 "special" sense have their own ganglionic centres, it 

 cannot be shown that the nervous fibres of "general" 

 sense, which either enter the cranium as part of the 

 cephalic nerves, or which pass up from the Spinal 

 cord, have any higher destination than the Thalami 

 Optici. So the motor fibres which pass forth from the 

 cranium, either into the cephalic nerve-trunks, or into 

 the motor columns of the spinal cord, though com- 

 monly designated as Cerebral, cannot be certainly said to have a higher 

 origin than the Corpora Striata. And we shall find strong physiological 

 as well as anatomical ground for the belief, that the Cerebrum has 110 com- 

 munication with the external world, otherwise than by its connection with 

 the Seusori-motor apparatus; and that even the movements which are 

 usually designated as " voluntary " are puly so as regards their original 

 source", the stimulus which calls the muscles into contraction being even 

 then immediately issued from the Cranio-Spinal axis, as it is in the move- 

 ments prompted by the reflex stimulation of an external impression. 



456. Wherever a Cerebrum is superimposed upon the Sensory Ganglia, 

 we find another ganglionic mass, the Cerebellum, superimposed upon the 

 Medulla Oblongata. The development of this organ bears a general, but 

 by no means a constant relation to that of the Cerebrum; for in the lowest 

 Fishes it is a thin lamina of nervous matter on the median line, onlv par- 

 tially covering in the "fourth ventricle;" whilst iu the higher Mammalia, as 

 in Man, it is a mass of considerable size, having two lateral lobes or hemi- 

 spheres in addition to its central portion. The direct communication which 

 the Cerebellum has with botli columns of the Spinal cord, and the com- 

 paratively slight comruissural connection which it possesses with the higher 

 portions of the Encephalic centres, justify the supposition that it is rather 

 concerned in the regulation and co-ordination of the actions of the former, 

 than in any proper psychical operations; and it will hereafter be shown that 

 the various kinds of evidence afforded by Comparative Anatomy, by Ex- 

 perimental inquiry, and by Pathological observation, all tend to support 

 this view of its function. 



457. Now although every segment of the Spinal Cord, and every one of 

 the Sensory Ganglia, may be considered, in common with the Cerebrum, as 

 a true and independent centre of nervous power, yet this independence is 

 only manifested when these organs are separated from each other; either 



