CHAP, ii.] THE BRAIN. 1013 



from this that in a normal condition of things afferent impulses 

 are continually passing up to the brain from the semicircular 

 canals, and that the loss of coordination which follows upon injury 

 to the canals is due to these normal impulses being deficient or 

 altered. It may be that such normal impulses do not exist, and 

 that the loss of coordination is the result of the central machinery 

 for coordination being interfered with by quite new impulses gene- 

 rated by the injury to the canal with the consequent loss of endo- 

 lymph acting as a stimulus to the endings of the nerve. For the 

 experience quoted above, though it proves that afferent impulses 

 other than those of sight, touch and the muscular sense do reach 

 the brain and afford a basis for a judgment as to the position 

 of the body, does not by itself prove that those impulses come 

 from the semicircular canals ; the arrangement of the canals is 

 undoubtedly suggestive ; but it is quite possible that the afferent 

 impulses in question may be generated by one or other of various 

 changes, vaso-motor and others, of the tissues of the body which 

 are involved in a change of position. And if it be true as affirmed 

 by some observers that both auditory nerves may be completely 

 and permanently severed, without any effect on the coordination 

 of movements, it is obvious that the incoordination which follows 

 upon section of the semicircular canals is due to some special 

 irritation set up by the operation and not to the mere absence of 

 any normal ampullar impulses. On the other hand, if the effects 

 are those of irritation, it is difficult to understand how they can, 

 as according to certain observers they certainly do, become per- 

 manent. It has however been strongly urged that in such cases 

 of permanent incoordination, the operation has set up secondary 

 mischief in the brain, in the cerebellum for instance, with which 

 as we have seen ( 618) the vestibular auditory nerve makes 

 special connections, and that the permanent effects are really due 

 to the disease going on here; and we have reason, as we shall 

 see, to think that the cerebellum is concerned in the coordination 

 of movements. It cannot therefore be regarded as settled that 

 the canals are the source of normal impulses, or that our conscious 

 appreciation of the position of the head and so of the body in 

 space is based on such impulses. But such a view is not dis- 

 proved ; and in any case it remains true that injury to the canals 

 does in some way or other, either by generating new impulses 

 or by altering preexisting ones, so modify the flow of 'afferent 

 impulses into the machinery of coordination as to throw that 

 machinery out of gear. 



643. We have dwelt on these phenomena of the semicircular 

 canals because they illustrate in a striking manner the important 

 part played by afferent impulses in the coordination of movements. 

 We saw reason to think ( 589) that even in an ordinary reflex 

 movement carried out by the spinal cord or by a portion of the 

 cord afferent impulses, other than those which excite the movement, 



