CHAP, ii.] THE BRAIN. 1015 



other facts suggest the view that the point at which the various 

 afferent impulses which form the basis of the sensations of a 

 conscious individual enter into the coordinating mechanism is or 

 may be some way short of the stage at which the complete 

 conversion of the impulse into a perfect sensation takes place. 

 The events which constitute what we may call visual impulses, as 

 these leave the retina to sweep along the optic nerve, are we must 

 admit very different from those which in the appropriate parts of the 

 brain constitute what we may call conscious vision; and probably 

 between the beginning 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 the imperfection 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 

 words the coordinating influences of sensory or afferent impulses 

 are not essentially dependent on the existence of a distinct 

 consciousness. 



644. We have raised this point partly for the sake of illus- 

 trating the working of the coordination machinery in the absence 

 of the cerebral hemispheres, but also in order to aid in the inter- 

 pretation of the subjective condition which we speak of as 

 giddiness or dizziness or vertigo. We compared the condition of 

 the pigeon after an injury to the semicircular canals 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 equili- 

 brium. 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 experience 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 associated with disease in the ear, being attributed by many 

 to disorder of the semicircular canals, and cases have been re- 

 corded 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 sensation and nothing else. In the well-known intense 

 vertigo which is caused by rapid rotation of the body visual 



