GENERAL SUMMARY. 



647 



Fig. 171. 



Brain of Cod: 



A, olfactive ganglia ; 



B, cerebral lobes ; 

 c, optic ganglia ; 

 D, cerebellum. 



680. In Man, however, as in all the higher Vertebrata, we find superimposed 

 (as it were) upon the Sensory ganglia, and constituting the 



principal mass of the Encephalon, the bodies which are known 

 as the Cerebral Hemispheres, or Hemispheric Ganglia. Now 

 when these are so greatly developed as to cover in and obscure 

 the Sensory ganglia to the degree which presents itself in Man, 

 it is not surprising that the fundamental importance of the 

 latter should not be generally recognized ; in Fishes, however, 

 the proportion between the two sets of centres is entirely re- 

 versed, the rudiments of the Cerebral Hemispheres (Fig. 171, 

 B) being usually inferior in size to the Optic ganglia (c) alone. 

 Indeed, of the pair of ganglionic masses to which that designa- 

 tion is usually applied, it may be 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, sepa- 

 rating the thin layer of true Cerebral substance which overlies 

 it, from the ganglionic mass which forms its floor. Between 

 these two extremes, a regular gradation is presented in the in- 

 termediate tribes. Now it is a point especially worthy of note, 

 that no sensory nerves 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 any or all of the nerve-fibres distributed to 

 the body, and the medullary substance of the Cerebrum. 

 For, whilst the nerves of " special" sense have their own gan- 

 glionic 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, 

 cannot be certainly said to have a higher origin than the Corpora Striata. 

 And we shall find strong physiological ground for the belief that the Cerebrum 

 has no communication with the external world, otherwise than by the Sensori- 

 motor apparatus which ministers to the automatic actions ; and that even the 

 movements which are usually designated as "voluntary" are only so as regards 

 their original source, the power which calls the muscles into contraction being 

 even then immediately derived from the Cranio-Spinal axis, as it is in the purely 

 automatic movements excited by an external impression. 



681. 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 j for in the lowest Fishes it is a thin 

 lamina of nervous matter on the median line, only partially covering in the 

 " fourth ventricle ;" whilst in the higher Mammalia, as in Man, it is a mass of 

 considerable size, having two lateral lobes or hemispheres in addition to its 

 central portion. The direct communication which the Cerebellum has with 

 both columns of the Spinal cord, and the comparatively slight commissural 

 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-ordi- 

 nation of the actions of the former, than in any proper psychical operations ; 

 and it will hereafter be shown that the evidence afforded by Comparative Ana- 

 tomy, by Experimental inquiry, and by Pathological observation, all tends to 

 support this view of its function. 



682. Now although every segment of the Spinal Cord and every one of the 



