SPECIAL CUTANEOUS SUBDIVISION. 127 



correspond very closely to those of the canal organs. (5) The 

 centers in which the nerves of the canal organs and ear end form 

 one continuous structure whose histology and secondary connections 

 in the different classes of vertebrates show that it has been derived 

 from the general cutaneous centers. 



The sense organs thus far spoken of lie in the semicircular 

 canals and in the sacculus and utriculus and are the only sense 

 organs of the ear in fishes. In addition to these there is developed 

 in higher vertebrates the spiral-shaped cochlea with its compli- 

 cated organ of Corti. The sense cells of this organ are also hair 

 cells which respond to vibrations in fluid, of a still higher rate. 

 Even this organ with its nerves and centers is so closely related 

 to the rest of the ear that it must be regarded as a more highly 

 specialized part of the same system of sense organs. 



The function of the system as a whole seems to have been at 

 first to aid an aquatic animal in directing its movements. Vibra- 

 tions in the water caused by surface waves, by the movements 

 of other animals, by its own movements, all these were capable 

 of stimulating these organs, and reactions to such stimuli con- 

 stituted the means by which the animal directed many of its move- 

 ments. Some of these organs becoming deeply imbedded in 

 canals were not directly influenced by large slow waves, but only 

 by the shorter quicker waves which could enter the canals. Some 

 of the canals becoming completely enclosed and being filled by a 

 denser fluid which responded more readily to waves of less amplitude 

 and higher rate, enabled the animal to be influenced by a wider 

 range of vibrations, including those of high enough rate to produce 

 sound. In this way the open pits, the canal organs, and the organs 

 of the ear came to be differentiated and to serve for a wide range 

 of stimuli. Finally a still higher and higher rate of stimuli were 

 provided for by the development of the cochlea, and as verte- 

 brates ceased to live an aquatic life the pit and canal organs which 

 responded to slow waves in water ceased to be of use and were 

 lost. 



The phenomenon of equilibration is a constant phase in the 

 control of movements. Indeed, keeping the equilibrium is a 

 necessary condition for all directed movement. It is probable 



