706 Or THE FUNCTIONS OF THE NERVOUS SYSTEM. 



of the gradual change in the relative development of the Sensory gan- 

 glia and Cerebral Hemispheres, which are presented to us in the Vertebrated 

 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. For 

 the sensory ganglia gradually increase, whilst the Cerebral Hemispheres 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 Amphioxus 

 or Lancelot, the lowest Vertebrated animal of which we have any knowledge, 

 in which there is not the rudiment of a Cerebrum, the Encephalon being only 

 represented by a single ganglionic mass, which, from its connection with the 

 nerves of sense, must obviously be regarded as analogous to the congeries of 

 ganglia that we find in the higher forms of the class. Descending to the In- 

 vertebrated series, we find that, except in a few of those which border most 

 closely upon Vertebrata (such, for example, as the Cuttle-Fish), the whole Cepha- 

 lic mass appears to be made up of ganglia in immediate connection with the 

 Nerves of Sense. These may appear to form but a single pair ; yet they are in 

 reality composed of several pairs, fused (as it were) into one mass. Of this we 

 may judge by determining the number of distinct pairs of nerves which issue 

 from them ; and also by the investigation of the history of their development, 

 the results of which bear a close correspondence with those obtained in the 

 preceding method. It is further to be remarked that the development of the 

 Cephalic ganglia in the Invertebrata always bears an exact proportion to the 

 development of the eyes; the other organs of special sense being comparatively 

 undeveloped; whilst these, in all the higher classes at least, are instruments 

 of great perfection, and are evidently connected most intimately with the 

 direction of the movements of the animals. Of this fact we have a remarkable 

 illustration in the history of the metamorphosis of Insects; the eyes being 

 almost rudimentary, and the Cephalic ganglia comparatively small, in most 

 Larvae; whilst both these organs attain a high development in the Imago, to 

 whose actions the faculty of sight is essential. 1 



733. Thus we are led by the very cogent evidence which Comparative Ana- 

 tomy supplies, to regard this series of Ganglionic centres as constituting the 

 real Sensorium ; each ganglion having the power of communicating to the mind 

 the impressions derived from the organ with which it is connected, and of excit- 

 ing automatic muscular movements in respondence to these sensations. If this 

 position be denied, we must either refuse the attribute of consciousness to such 

 animals as possess no other encephalic centres than these ; or we must believe 

 that the addition of the Cerebral hemispheres, in the Vertebrated series, alters the 

 endowments of the Sensory ganglia an idea which is contrary to all analogy. 



734. So far as the results of Experiments can be relied on, they afford a cor- 

 roboration of this view. The degree in which animals high in the scale of or- 

 ganization can perform the functions of life, without any other centre of action 

 than the Ganglia of Special sense, the Medulla Oblongata, and the Cerebellum, 

 appears extraordinary to those who are accustomed to regard the Cerebral Hemi- 

 spheres as the centre of all energy. From the experiments of Flourens, a 

 Hertwig, 3 Magendie, 4 Longet, 5 and others, it appears that not only Reptiles, but 



1 See "Princ. of Phys., Gen. and Comp.," CHAP. xx. Sect. 2, An^. Ed. 



2 " Recherches Experimentales sur les propriete's et les fonctions du Systeme Nerveux," 

 2dEdit. 1845. 



3 "Exper. de effect, laosion. in partibus Encephali," Berol., 1826. 



4 "Lec,ons sur les Fonctions du Systeme Nerveux," Paris, 1839. 

 6 "Traite de Physiologic," torn. ii. partie 2. 



