734 MACHINERY OF COORDINATION. [Book hi. 



about with difficulty or not at all, when the eyes are shut; 

 nevertheless there are reasons for thinking that it is in part 

 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 ac- 

 count 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 oscil- 

 lating 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 coordinating 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 in- 

 ability 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 disar- 

 rangement 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 movements are the 

 outcome, one subjective and the other objective, of the same 

 thing. It is not because Ave 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. 



VVe 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 

 curtailing 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 imper- 

 fect, but the central machinery is not thereby affected, though 

 its area of usefulness is limited, and no giddiness is experienced; 

 and so in other instances. 



