SENSE OF HEAKING. 875 



The oculo-motor nerve supplies all of the muscles of the eye with 

 the exception of the external rectus and the superior oblique. The ex- 

 ternal rectus muscle is supplied by the abducens, or sixth pair, while the 

 superior oblique is supplied by the patheticus or fourth pair. 



C. THE SENSE OF HEAKING. 



The sense of hearing or audition ma}^ be defined as that sense by 

 which the mind takes cognizance of the undulations of the elastic 

 medium which give rise to the sensation of sound. The mind recog- 

 nizes not the body producing the sound, but the impression made by its 

 vibrations upon the sensorium. Sound, then, does not take place in the 

 ear, but in the brain. When we speak of a resounding string or a 

 sonorous bell we make a mistake ; the string and the bell simply 

 vibrate. These vibrations give rise to undulations in the elastic ether, 

 which undulations are transmitted in various directions, and finally 

 through the auditory passages impress the auditory nerve and give rise 

 to sound. The physiological aspects of the question are here alone 

 regarded. Regarding sound as the recognition of the impressions 

 made by vibrating sonorous bodies on the auditory nerve, if every one 

 were deaf there would, of course, be no sound; but, on the other hand, 

 we say that no sound may be produced in a vacuum, although we know 

 that sonorous vibrations still take place ; hence we are also permitted in 

 a physical sense to give the above explanation of the word sound. 



The simplest form of the organ of hearing is a sac filled with fluid, 

 in which the ends of the auditory nerve terminate. In all groups of 

 animals the essential part of the organ of hearing consists in a certain 

 special form of termination of the nerve, which is alone capable of 

 receiving auditory impressions and of transmitting them to the central 

 ganglia of the brain. The more highly complicated forms of auditory 

 apparatus simply depend upon modifications which assist in the trans- 

 mission of sound to this part. 



In all the invertebrates the organ of hearing is restricted to such a 

 simple sacular form, in some instances containing otoliths, or small, hard 

 granules, and the vibrations of the sonorous body are communicated to 

 the nerve of hearing spread over this sac. In such animals it is prob- 

 able that while a nerve of hearing is present only simple sounds and 

 noises can be recognized, while no difference in pitch or intensity can be 

 distinguished. Such a simple form of apparatus, consisting simply of 

 liquid contained in a sac, is found in mollusks. 



In crustaceans there is a rudimentary organ of hearing present, 

 placed on each side of the base of the anterior antennae. It likewise 

 consists simply of a membranous sac filled with fluid, and on which 

 ramify the fibres of the nerve of hearing. 



