646 THE BRAIN. 



they have been moved. There can be no doubt that such a judgment is 

 based upon the interpretation by consciousness of afferent impulses which 

 are dependent on the position of the body, but which are not afferent im- 

 pulses belonging to sensations of touch or sight, or taking part in the mus- 

 cular sense. And it is urged with great plausibility that the afferent im- 

 pulses in question are those which we have just referred to as started in the 

 semicircular canals. 



If we admit the existence of such ampullar impulses, if we may venture 

 so to call them, and recognize them as contributing largely not only to our 

 direct perception of the position of the head and thus of the body, but also 

 in a more indirect way to what we have called the sense of equilibrium, we 

 should expect to find that when they are abnormal the sense of equilibrium 

 is disturbed, and that in consequence a failure of coordination in our move- 

 ments results. And the loss of coordination which we described above as 

 resulting from injury to the semicircular canals has accordingly been attrib- 

 uted to a deficiency or disorder of normal ampullar impulses. 



But we must here distinguish between two things. It seems clear that 

 when the membranous canals are injured or otherwise stimulated, afferent 

 impulses are generated which, on the one hand, may produce peculiar move- 

 ments of the head, and, on the other hand, seem able when the injury is large 

 to cause a loss of coordination of bodily movements. But it does not neces- 

 sarily follow 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 generated by the injury to the canal with the consequent loss of 

 eudolymph acting as a stimulus to the endings of the nerve. For the expe- 

 rience 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, 

 vasomotor 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 incoordi- 

 nation which follows upon section of the semicircular canals is due to some 

 special irritation setup 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 permanent. 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 ( 531), the vestibular auditory nerve makes 

 special connections, and. that the permanent effects are really due to the dis- 

 ease 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 

 disproved ; 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 pre- 



