SUPPORT, MOTION, AND SENSATION 



361 



Ears 



The structures making up the compUcated mechanism of iiearing 

 primarily serve two purposes, namely equilibration and hearing. Of 

 these functions the first is luidoubtedly the more primitive. 



Most invertebrates, whether jellyfish, molluscs, or crayfish, main- 

 tain their equilibrium by some sort of otocyst. Roughly described, 

 this consists of a sac lined throughout or in part by cell-receptors and 

 containing concretions called otoliths. As the animal changes its 

 position the otolith shifts due to the forces of gravity and thus stimu- 

 lates by contact the different receptor nerve cells, which transmit the 

 impulse of pressure to the ^ ^ 



endolymphatic 

 Sac 



endolymphcctic duct 



anter-ior 



Semicircular-/ 



CxxnaL 



posterior- 

 Semicircular- 

 Corjal 



utriculLcs 



brain, where it is interpreted 

 so as to enable the animal to 

 right itself. 



The ecjuilibratory mecha- 

 nism of vertebrates functions 

 principally through stimuli 

 received from nerve cells 

 located in the arnqmllae or 

 swollen ends of three semi- 

 circular canals, occupying 

 roughly the three planes of 

 space. The animal is enabled 

 to adjust its position wdth 

 reference to the stimuli re- 

 ceived through the influence 

 of gravity. In such cases the 

 fluid within the semicircular 

 system stimulates differen- 

 tially the nerve endings in 

 the ampullae. Stimuli reach 

 the nerve-receptors in the same manner as they do in the lower 

 forms, being carried by branches of the auditory nerve (VIII) in the 

 brain. The entire structure is protected by a surrounding mass of 

 cartilage which in higher forms becomes ossified. 



As to the function of hearing, it is possible that in the case of 

 fishes vibrations are transmitted by the water through the skull to 

 the sensory inner ear. However, when air is substituted for water 

 as the chief environment some other more sensitive device must be 



'hor-L3onta.l 



Semicirculctr- 



Cocnccl 



The inner ear of a fish showing the essential 

 features of this balancing? organ. Where are 

 the ampullae!' These, together with areas in 

 the alriculus and sacculus, contain patches of 

 sensory cells connected with branches of the 

 auditory nerve. How are such areas stimu- 

 lated:* The lagena produces the cochlea. 



