616 THE SEMI-CIRCULAR CANALS. [BOOK in. 



judgment as to the angle through which they have been moved. 

 What are the data on which such a judgment can be formed? 

 It is possible that the mere displacement of blood or of the more 

 fluid parts of the tissues in various regions of the body, by giving 

 rise to affections of general sensibility, may contribute to these 

 data ; but the peculiar features of the semi- circular canals suggest 

 that these are special agents in this matter. The three canals are, 

 as we know, placed in the head in planes nearly at right angles to 

 one another. Hence the pressure of the endolymph on the walls 

 of the canal (including the maculae of the ampullae) in any given 

 position of the head, and variations of that pressure due to move- 

 ments of the head, or the movements of the endolymph within the 

 canals accompanying movements of the head, would be different 

 in the three canals; a sonorous wave on the other hand would 

 affect all the ampullae equally. If we suppose that the pressure of 

 the endolymph or variations in that pressure, or the movements of 

 the endolymph can give rise to afferent impulses which, though 

 passing up to the brain along the auditory nerve, are not of 

 the nature of auditory impulses, we appear to have the data for 

 which we are seeking; for it is quite possible to conceive that 

 the impulses thus generated in the ampullae by movements of 

 the head, should by becoming transformed into sensations enter 

 into the judgment which we form, of the movements which have 

 given rise to them. 



But if ampullar sensations, if we may so call them, thus enter 

 into our appreciation of the position of our body and thus form, in 

 part, the basis of our sense of equilibrium, it is obvious that when 

 these are absent or deranged, the sense of equilibrium will be 

 affected and the coordination of movements interfered with. And 

 it has been urged that the phenomena attendant on injury to the 

 semi-circular canals are due either to the absence of normal or to 

 the influence of abnormal ampullar sensations. There are how- 

 ever difficulties in the way of giving a satisfactory explanation of 

 these phenomena. If, as some observers state, 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 auditory canals is 

 due to some irritation set up by the operation and not to the 

 absence of any normal impulses passing up from those organs to the 

 brain, to the lack of what we called just now ampullar sensations. 

 But 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, and 

 that the permanent effects are really due t6 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. But the matter is 



