344 ZOOLOGY FOR MEDICAL STUDENTS chap. 



render possible the analysing of a movement of rotation into its com- 

 ponents in these planes and its detection whatever its direction may be. 



The cavity of the otocyst including the canals is filled with a watery 

 fluid — the endolymph. Suppose the Dogfish whose left otocyst is pictured 

 in Fig. 143 as seen from a point away on the animal's left turns sharply 

 to the right, then it is clear that the external semicircular canal will be 

 carried round in its own plane in the direction indicated by the left side 

 of the page. As the endoljntnph within the canal possesses inertia the 

 lining of the canal will as it were rub over the surface of the stationary 

 fluid. But certain of the lining cells — in the ampulla — possess large 

 sensory hairs which project freely into the endolymph. These as they 

 are dragged through the fluid will be bent and stimulated just exactly in 

 the same way as if they were stationary and the fluid were rushing over 

 them in the opposite direction. In this way a sensation of turning to 

 the right side is produced. It is clear that rolling movements of the 

 body will similarly be detected by the vertical canals and that in fact 

 any rotatory movement will give rise to a sensation by the combination 

 of effects on the different canals. 



Another point to notice is that prolonged turning movement say to 

 the right will overcome the inertia of the endolymph. This latter will 

 be set in motion and then if the turning movement be suddenly stopped 

 the inertia of the endolymph will keep it moving for some little time 

 and, rushing over the sensory hairs, it will produce a sensation the same 

 as if the body were rapidly turning in the opposite direction. It is in 

 this way that dizziness is produced when a turning movement continued 

 for some time is suddenly stopped. 



From the lower corner of the •otocyst there projects a bluntly pointed 

 pocket — ^the lagena (Fig. 143, Z), with sensory cells of a somewhat different 

 type in its lining. This lagena marks the first appearance in evolution 

 of a portion of otocyst devoted to a new sense — ^that of hearing— and it 

 is destined during the evolution of the higher vertebrates, to become an 

 organ of great complexity known as the cochlea. 



There are two points that should be noted in regard to which the 

 otocyst of the Dogfish and its allies differs from that of the higher verte- 

 brates. In the latter the body of the otocyst becomes deeply constricted 

 across into a dorsal portion the utriculus, with which the canals are 

 connected, and. a ventral portion the saceulus, which carries the lagena. 

 In the Dogfish the division of the otocyst into those two parts is less 

 complete, the two cavities still communicating by a long slit-like opening. 

 Further, in vertebrates above the Dogfish group the tubular connec- 

 tion with the outer surface becomes nipped across and the tubular 



