98 MOTIONS OF THE EYEBALLS. 



of a voltaic current through the brain (cerebellum ?), in 

 which case the direction of the subjective rotation is 

 determined by that of the current. Both kinds of vertigo 

 are accompanied by nystagmus. In the frog, after 

 removal of the hemispheres, locomotion is as perfect as 

 in the normal animal, provided that the optic lobes and 

 cerebellum are present. A frog which possesses its cere- 

 bellum, but has no optic lobes, jumps normally, but fails 

 in maintaining its balance. In mammalia (the dog), loss 

 of the cerebellum is followed by disorders and perverted 

 action of the muscles of the trunk and limbs, in con- 

 sequence of which, neither voluntary nor reflex acts 

 can be accomplished in a manner adapted to their pur- 

 pose. 



All of these phenomena may be understood on the 

 supposition that centres exist (in the cerebellum ?) of 

 of which it is the function (i) to receive impressions as 

 to the direction of the passive and active motions of the 

 body and of its parts ; and (2) to direct and regulate the 

 actions of the muscles of the trunk and limbs (indepen- 

 dently of consciousness) in obedience to those impres- 

 sions. 



Regulation of the Motions of the Eyeballs and the 

 Actions of the Iris and Tensor of the Choroid. These 

 actions are governed by centres which have their seat in 

 the floor of the aqueduct of Sylvius, and of the 3rd and 

 4th ventricles, the influence of which is conveyed to the 

 muscular structures, over which they preside, by the 3rd, 

 4th, and 6th nerves, as well as by channels which are con- 

 tained in the cervical portion of the spinal cord, and in 

 the corresponding part of the sympathetic system. They 

 have been localized by experiment as follows : Excita- 

 tion of the floor of the aqueduct at its entrance into the 

 third ventricle, produces convergence of the visual axis, 

 and contraction of the pupil ; excitation of the anterior 

 tubercles of the corpora quadrigemina, or of the optic 



