1016 MACHINERY OF COORDINATION. [BOOK HI. 



sensation plays a part when the rotation is carried on with the eyes 

 open, but only a part ; for vertigo may be induced, though not so 

 readily, by rotation with the eyes completely shut. In the latter 

 case it has been suggested that the vertigo is caused by abnormal 

 ampullar impulses, but these can only contribute to the result 

 which is in the main caused by direct disturbance of the brain. 

 When the rotation is carried out with the eyes open, the vertigo 

 which is felt when the rotation ceases is partly caused by the 

 visual sensations, on account of the behaviour of the eyeballs, 

 ceasing to be in harmony with the rest of the sensations and 

 afferent impulses which help to make up the coordination. The 

 rotation sets up peculiar oscillating movements of the eyeballs, 

 which continue for some time after the rotation has ceased ; owing 

 to these movements of the eyeballs the visual sensations excited 

 are such as would be excited if external objects were rapidly 

 moving, whereas all the other sensations and impulses which are 

 affecting the central nervous system are such as are excited by 

 objects at rest. In a normal state of things the visual and the 

 other sensations and impulses, which go to make up the coordina- 

 ting machinery, are in accord with each other in reference to the 

 events in the external world which are giving rise to them ; after 

 rotation they are for a time in disaccord, and the coordinating 

 machinery is in consequence disarranged. 



When we interrogate our own consciousness, we find that we 

 are not distinctly conscious of this disaccord ; the visual sensations 

 are so prepotent in consciousness, that we really think the external 

 world is rapidly whirling round ; all that we are further conscious 

 of is the feeling of giddiness and our inability to make our bodily 

 movements harmonize with our visual sensations. So that even in 

 the cases where the loss of coordination is brought about by 

 distinct sensations what we really appreciate by means of our 

 consciousness is the disarrangement of the coordinating machinery. 

 It is the appreciation of this disorder which constitutes the feeling 

 of vertigo ; both the feeling of giddiness and the disordered move- 

 ments are the outcome, one subjective and the other objective, of 

 the same thing. It is not because we feel giddy that we stagger 

 and reel ; our movements are wrong because the machinery is at 

 fault, and it is the faulty action of the machinery which also makes 

 us feel giddy. 



We may here perhaps remark that it is an actually disordered 

 condition of the coordinating mechanism which gives rise to the 

 affection of consciousness which we call giddiness, not a mere cur- 

 tailing of the mechanism or any failure on its part to make 

 itself effective. Complete blindness limits the range of activity 

 of the machinery but leaves the remainder intact, and no giddi- 

 ness is felt. So again in certain diseases of the nervous system 

 the muscular sense is interfered with over considerable regions 

 of the body, and in these regions coordination fails or is imperfect, 



