FUNCTIONS OF THE SENSORY GANGLIA. 511 



ments whereby sensations, of whatever kind, either originate or direct 

 Automatic movements. 



903. So far as the results of experiments can be relied on, they 

 afford a confirmation of these views, by showing that sensory impressions 

 can be felt, and that automatic movements of a higher kind than the 

 simply reflex can be called into play after the removal of the Cerebral 

 Hemispheres, provided that these ganglia be left intact. Thus, if a 

 Bird be thus mutilated, it maintains its equilibrium, and recovers it 

 when it has been disturbed ; if pushed, it walks ; if thrown into the air, 

 it flies. A pigeon deprived of its cerebrum has been observed to seek 

 out the light parts of a partially-illuminated room in which it was con- 

 fined, and to avoid objects that lay in its way; and at night, when 

 sleeping with closed eyes and its head under its wing, it raised its head 

 and opened its eyes upon the slightest noise. So, again, the removal 

 or destruction of one pair of these Sensory centres appears to involve 

 the loss of the particular sense to which it ministers ; and frequently, 

 also, to occasion such a disturbance in the ordinary movements of the 

 animal, as to show the importance of these centres in regulating them. 

 Such experiments have been chiefly made upon the Optic ganglia, or 

 Corpora Quadrigemina, the partial loss of which on one side produces 

 temporary blindness in the eye of the opposite side, and partial loss of 

 muscular power on the opposite side of the body ; and the removal of a 

 larger portion, or the complete extirpation of it, occasions permanent 

 blindness and immobility of the pupil, and temporary muscular weak- 

 ness, on the opposite side. This temporary disorder of the muscular 

 system sometimes manifests itself in a tendency to move on the axis, as 

 if the animal were giddy ; and sometimes in irregular convulsive move- 

 ments. Here, then, we have proof of the necessity of the integrity of 

 this ganglionic centre for the possession of the sense of vision ; and we 

 have further proof that the ganglion is connected with the muscular 

 apparatus, by motor nerves issuing from it. The reason why the eye 

 of the opposite, side is affected is to be found in the decussation of the 

 optic nerves, a point to be immediately adverted to ( 907). The 

 influence of the operation on the muscles of the opposite side of the body 

 is at once understood from the fact of the decussation of the motor 

 fibres in the anterior pyramids ( 890). Similar disturbances of move- 

 ment have been produced by injuries to the organs of sense themselves, 

 or to the nerves connecting them with the sei^sorral centres. Thus, if 

 one of the eyes of a pigeon be blindfolded, or its humours be evacuated, 

 vertiginous motions ensue ; and section of one of the semicircular canals 

 of the ear in pigeons and rabbits has been found to occasion constant 

 efforts to move in the plane of that canal, thus confirming the belief 

 that the function of these canals is to indicate the direction of sounds 

 (8 952). 



904. Notwithstanding that, in Man, the high development of Intel- 

 ligence, and the exercise of the Will, supersede in great degree the 

 operations of Instinct, we still find that there are in ourselves certain 

 movements which can be distinguished as neither voluntary nor simply 

 reflex, and which are examples of the method of operation that seems to 

 be the chief source of the actions of the lower Vertebrata, as of the 



