1202 THE EAR. 



the posterior canal of the other. Crum Brown states the matter 

 thus : 



"Further, in each of these three pairs (right and left horizontal, right 

 superior and left posterior, right posterior and left superior), the two canals 

 are so placed that when rotation takes place about the axes to which they are 

 perpendicular, one of the two canals moves, with its ampulla preceding the 

 canal, so that the flow or tendency to flow [or pressure] is from ampulla to 

 canal, while in the other the ampulla follows the canal, and the flow or tend- 

 ency to flow [or pressure] is from canal to ampulla. If, then, we suppose that 

 flow from ampulla to canal or, adopting Mach's view, increase, of pressure in 

 the ampulla alone stimulates the hair cells, while no effect is produced by 

 flow in the opposite direction or by diminution of pressure in the ampulla 

 we have in the six canals a mechanical system capable of giving us an accurate 

 notion of the axis about which rotation of the head takes place, and of the 

 sense of rotation." 



Cyon, 1 in later researches, has performed certain experiments which, 

 it must be confessed, throw doubt on both the statical and dynamical 

 theories already described. He has punctured the canals and drawn 

 off the endolymph, injected the canals with gelatin, and plugged them 

 with slender filaments of laminaria, which soon became swollen. Yet 

 by these severe proceedings, by which great variations of pressure must 

 be produced, none of the phenomena of Flourens were observed. The 

 obvious objection to these experiments is, that they were so rough as to 

 entirely destroy the mechanism they were intended to examine. Again, 

 Cyon has repeatedly observed, after making a minute puncture in a 

 canal, that there was a minute pulsation, caused either by a small artery 

 supplying the canal, or by the pulsations of the brain itself through the 

 fluid in the arachnoid with which the membranous canals communicate ; 

 and he suggests that, if Mach's view be correct, we ought in this way to 

 have rhythmic excitations of the nerve-endings in the ampulla. As it 

 is not likely, however, that such pulsations would have any effect, unless 

 the canal were punctured, the observation is of no great importance ; at 

 all events, it is not a valid objection to Mach's theory. 



It is well known that a rabbit may show the phenomena of Purkinje 

 after having been rotated on a turn-table. Cyon took advantage of 

 this fact and performed the experiment on animals in which the auditory 

 nerves had been divided, and he states that the results were the same 

 as in normal animals. This observation would at first sight appear to 

 seriously compromise the statical and dynamical theories, but what it 

 really does is to show that some of the phenomena of Purkinje may, as 

 he himself supposed, be due to the action of rotation on the brain itself. 

 If so, one would expect to see these phenomena after the division of the 

 auditory nerves, and the fact does not invalidate the reasoning on 

 theoretical and experimental data regarding the function of the canals 

 already described. 



Cyon contends that the semicircular canals only indirectly assist in 

 giving a knowledge of spatial relations. He holds that knowledge of 

 the position of bodies in space depends on nervous impulses coming 

 from the contracting ocular muscles; that the oculomotor centres are 

 in intimate physiological relationship with the centres receiving im- 

 pulses from the nerves of the semicircular canals ; and that the oculo- 

 motor centres, thus excited, produce the movements of the eyeballs, 



1 These, op. cit., pp. 41, 42, 43. 



