> 44 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 endolymph 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 

 arc 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 

 anil, 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, 1), with sensory cells of a somewhat different 



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



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



-lined during the evolution of the higher vertebrates to become an 



) 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- 



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



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



oimected, and a ventral portion the sacculus, which carries the lagena. 



I" ' i the division of the otocyst into those two parts is less 



;>lete, the two cavities still communicating by a long slit-like opening. 



Kurt hrr, in vertebrates above the Dogfish group the tubular connec- 



\vith the outer surface becomes nipped across and the tubular 



