G48 THE BRAIN. 



stitute what we may call conscious vision ; and probably between the begin- 

 ning and the end there are progressive changes. It is probable, we say, that 

 these visual events may affect the coordinating mechanism at some stage of 

 their progress before they reach their final and perfect form. If this be so 

 we may further conclude that though, when the whole nervous machinery is 

 present in its entirety, the afferent impulses which take part in coordination 

 must inevitably at the same time give rise to conscious sensations, they 

 might still effect their coordinating work when, owing to their imperfec- 

 tion or lack of the terminal part of the nervous machinery, the impulses 

 failed to receive their final transformation, and conscious sensations were 

 absent. In other \vords, the coordinating influences of sensory or afferent 

 impulses are not essentially dependent on the existence of a distinct con- 

 sciousness. 



557. We have raised this point partly for the sake of illustrating the 

 working of the coordination machinery in the absence of the cerebral hemi- 

 spheres, but also in order to aid in the interpretation of the subjective condi- 

 tion which we speak of as giddiness or dizziness or vertigo. The condition 

 of the pigeon after an injury to the semicircular canals is comparable to that 

 of a person who is giddy or dizzy, and, indeed, vertigo is the subjective 

 expression of a disarrangement of the coordination machinery, especially 

 of that concerned in the maintenance of bodily equilibrium. It may be 

 brought about in many ways. When a constant current of adequate strength 

 is sent through the head from ear to ear, we experienced a sense of vertigo ; 

 our movements then appear to a bystander to fail in coordination, in fact to 

 resemble those of a pigeon whose semicircular canals have been injured ; 

 and, indeed, the effects are probably produced in the same way in the two 

 cases. In what is called Meniere's disease attacks of vertigo seem to be asso- 

 ciated with disease in the ear, being attributed by many to disorder of the 

 semicircular canals, and cases have been recorded of giddiness as well as 

 deafness resulting from disease of the auditory nerve. Visual sensations are 

 very potent in producing vertigo. Many persons feel giddy when they look 

 at a waterfall ; and this is a case in which both the sense of giddiness and 

 the disarrangement of coordination is the result of the action of a pure sen- 

 sation and nothing else. In the well-known intense vertigo which is caused 

 by rapid rotation of the body visual 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 am- 

 pullar 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 car- 

 ried 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 behavior of the 

 eyeballs, ceasing to be in harmony with the rest of the sensations and affer- 

 ent 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 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 dis- 



