618 VERTIGO. [BOOK in. 



nerves, it is clear that the semi-circular canals can have little share 

 in this form of vertigo. And indeed, even admitting this as a contri- 

 bution to the total effect, it seems probable that changes in the brain 

 due to the displacement of the blood or even of the brain-substance 

 itself caused by the too rapid rotation, are at work. It is difficult 

 otherwise to explain the unconsciousness which may ensue if the 

 rotation be rapid and long-continued ; and the vertigo resulting 

 from various poisons seems to be distinctly of central origin. 



Whether we accept the view of ampullar sensations just dis- 

 cussed or not, and whatever be the exact share which false ampul- 

 lar sensations take in the causation of vertigo, this at all events is 

 clear, that afferent impulses of various kinds so far contribute to 

 the building up of the coordinating mechanisms that changes in 

 these impulses tend to throw the mechanisms into disorder, or at 

 least to impair their proper working. It is not necessary that 

 these afferent impulses should directly affect consciousness (or, to 

 speak more correctly, should affect that complete consciousness 

 which is associated with volition), and so develope into distinct 

 perceptions. We have seen that a bird from which the cerebral 

 hemispheres have been removed is perfectly able to fly ; and that 

 therefore the coordinating nervous mechanism necessary for flight 

 is situated in the parts of the brain lying behind the cerebral 

 hemispheres. We have also dwelt on the fact that all the chief 

 coordinating mechanisms of the frog lie in the hind parts of the 

 brain ; yet in the frog, as in the bird, and we may add, as in the 

 mammal, injury to the hinder parts of the brain produces loss 

 of coordination whether the hemispheres be present or not. Now, 

 we have no satisfactory reasons for either asserting or denying that 

 what we call consciousness, i.e. a distinct consciousness similar to 

 our own consciousness, exists in animals deprived of their cerebral 

 hemispheres. When signs of volition are present, we may safely 

 take these signs as indications of consciousness also ; but we are not 

 justified in saying that all consciousness is absent when satisfactory 

 signs of volition are wanting. We cannot form any just judgment 

 on the matter without some more trustworthy and objective tokens 

 of consciousness than we at present possess. But what we may 

 safely assert is, that the coordinating mechanism, the retention of 

 which is so striking a feature of an animal deprived of its cerebral 

 hemispheres, is constructed out of divers afferent impulses of 

 various kinds arriving at the coordinating centre from various 

 parts of the body, that in fact the coordination taking place at the 

 centre is the adjustment of efferent to afferent impulses. Many, if 

 not all, of these afferent impulses are such that in the presence of 

 consciousness they would give rise to perceptions and ideas ; but 

 we have no reason for thinking that the complete development of 

 the afferent impulse into a perception or an idea is always necessary 

 to the carrying out of coordination. We may say that we have a 

 sense of equilibrium by means of the semi-circular canals, and 



