THE SEMICIRCULAR CANALS. 1203 



which then determine our notions of spatial relations. The canals are 

 thus the peripheric organs of the sense of space, and the sensations 

 caused by excitation of the nervous terminations in the ampullae assist 

 in forming our notions of the three dimensions, the sensations excited 

 by each canal corresponding to one dimension. Cyon, however, does 

 not show how the ampullae are excited in the first instance. Further, 

 the mechanism assumed to exist is much more complicated than that 

 required for the static or dynamic theories. The latter explain in a 

 satisfactory manner the cause of the sensation of rotation, without 

 touching the much larger question of the origin of our notions of space ; 

 and whether one adopts, with regard to this problem, the intuitional 

 or empiric theories, there remains the fact that a comparatively simple 

 function has been suggested for the canals. The interpretation of 

 spatial relations does not, in all probability, depend on one organ alone, 

 but on the combined effect of nervous impulses coming from many 

 organs — eye, ear, skin, tendons, and muscles ; and to say that the canals 

 are the only organs having to do with our knowledge of spatial relations 

 is to present a fragment of the truth. 



The view of Cyon as to the influence of the canals on the move- 

 ments of the eyes, is supported by some recent experiments carried out 

 by Lee. 1 By passive rotation of a dog-fish round different axes, com- 

 pensating movements of the eyes and fins (especially the pectoral fins) 

 were produced. It was observed that if the fish were rotated in the 

 plane of one of the canals, exactly the same movements of the eyes and 

 fins occurred as were produced by experimental operation and stimula- 

 tion of the ampulla of that canal. Lee is of opinion that each ampulla 

 has a principal and a subordinate function, which produce opposite 

 effects on the eye movements. Thus, if the chief function of one 

 ampulla is to move the eyeball upwards, the subordinate effect is to 

 rotate it downwards. By pressing with a blunt needle upon the ampulla, 

 either of those functions may be called forth. For example, gentle 

 pressure upon the anterior ampulla causes the eyeball of the same side 

 to roll slightly downwards, and that of the opposite side slightly up- 

 wards. This is the subordinate function. A stronger pressure upon 

 the same ampulla causes the eyeball of the same side to roll strongly 

 upwards, and that of the opposite side to roll strongly downwards — this 

 is the principal function. It would appear that the compensatory 

 movements of the eyes and fins, when the fish is rotated, are produced 

 by the principal function of a pair of canals, while the reactions follow- 

 ing the compensatory movements are produced by the subordinate 

 functions of a pair of canals. On electrically stimulating different parts 

 of the cut surface of the auditory nerves, results were obtained the same 

 as when the stimulus was applied to the corresponding ampulla, and the 

 reverse of the results obtained by mere section of the same nerve. 

 Further, when rotation occurs in a plane different from that of any of 

 the canals, two pairs of canals are stimulated to varying degrees by the 

 pressure of the endolymph, and the result corresponds to the algebraic 

 sum of the stimuli to the various ampullae. These striking results 

 strongly support the dynamic theory. 



The researches of Ewald 2 merit attention, as he has been led to 

 extend the influence of the canals. He divides the labyrinth function- 



1 Journ. Physiol., Cambridge and London, vol. xvii. p. 192. 



- "Untersuch. u. das Endorgan des Nervus octavus," Wiesbaden, 1892. 



