474 



ORGANIC SENSATIONS 



and crocodiles. In the last named for the first, and in birds the cochlea 

 becomes curved and slightly spiral, while in the mammals it reaches its highest 

 development by growing out into a long tube which describes upon itself 

 one and one-half to four spiral turns. 



The semicircular canals are arranged in the three dimensions of space. 

 Inasmuch as the physiological investigations of these structures relate mainly 

 to the pigeon, we shall describe them for this animal at once, following the 

 work of J. K. Ewald. We find on each side of the head an external, an 

 anterior and a posterior canal (Figs. 184 and 185). The two external canals 

 lie almost exactly in the same plane, which when the head is in its normal 

 position with the beak slightly lowered, is approximately the horizontal plane 

 (Fig. 184). The planes of the posterior canal of one side and the anterior 

 canal of the other are almost exactly parallel, but the projection of each is 

 about 7 mm. distant from the other (Fig. 186) and each forms an angle of 

 about 45 with the median vertical plane of the head. This relationship being 



b 



FIG. 184. The semicircular canals of the pigeon laid bare in situ, after J. R. Ewald. 

 a, b and c are placed in the axes of the eyes, the skull and the beak. 



The rods 



true for both pairs (the left anterior with the right posterior, and the right 

 anterior with the left posterior), it follows that the six canals together mark 

 out three planes which lie in the three dimensions of space. 



This description applies strictly only to the middle portion of each canal, 

 for its ends deviate somewhat from the course taken by the middle. 



Each canal bears at one end an enlargement, the ampulla, which contains 

 in its crista acustica the nerve endings of the canal. The ampullae of the two 



