CHAP. IL] THE BRAIN. 1019 



sensation that the ground in front of them is suddenly sinking 

 away beneath their feet. Without trusting too closely to the 

 interpretations the subjects of these disorders give of their own 

 feelings, and remembering what was said above concerning vertigo, 

 we may at least conclude that the unusual movements are in many 

 cases due to a disorder of the coordinating mechanism, brought 

 about by strange or disordered sensory impulses. And this view 

 is supported by the fact that many of these forced movements 

 are accompanied by a peculiar and wholly abnormal position of 

 the eyes, which alone might perhaps explain many of the pheno- 

 mena. 



646. The phenomena presented by animals deprived of f 

 their cerebral hemispheres shew that this machinery of coordina- 

 tion is supplied by cerebral structures lying between the cerebral 

 hemisphere above and the top of the spinal cord below. But 

 when we ask the further question, how is this machinery related 

 to the various elements which go to make up this part of the 

 brain ? the only answers which we receive are of the most 

 imperfect kind. 



In the case of the frog we can, after removal of the cerebral 

 hemispheres, make an experimental distinction in the parts left 

 between the optic thalami with the optic nerves and tracts, 

 the optic lobes, and the bulb with the rudimentary cerebellum. 

 When the optic thalami are removed, as might be expected, the 

 evidence of visual impressions modifying the movements of the 

 animal disappears ; and it is stated that apparently spontaneous 

 movements are much more rare than when the thalami are 

 intact. When the optic lobes as well as the cerebral hemispheres 

 are removed, the power of balancing is lost ; when such a frog is 

 thrown off its balance by inclining the plane on which it is placed, 

 it slips back or falls down ; the special coordinating mechanism 

 for balancing must therefore in this animal have a special 

 connection with the optic lobes. But after removal of these 

 organs the animal is still capable of a great variety of coordinate 

 movements : unlike a frog retaining its spinal cord only, it can 

 swim and leap, it maintains a normal posture, and when placed on 

 its back immediately regains the normal posture. The cerebellum 

 of the frog is so small, and in removing it injury is so likely to be 

 done to the underlying parts, that it becomes difficult to say how 

 much of the coordination apparent in a frog possessing cerebellum 

 and bulb is to be attributed to the former or to the latter ; 

 probably, however, the part played by the former is small. 



In the case neither of the bird nor of the mammal have we 

 any exact information as to the behaviour of the animal after 

 removal of the parts behind the hemispheres, in addition to the 

 hemispheres themselves. Our knowledge is confined to the 

 results of the ablation, or of the stimulation of parts, the 

 cerebellum for instance, in animals in which the rest of the brain 



