THE SENSE OF HEARING 719 



elongated, and project outward, forming a reticulated membrane through 

 ;he meshes of which the hair-like processes of the cells project. 



From the connective-tissue thickening on the upper surface of the os- 

 seous lamina spiralis there extends outward over the organ of Corti a thin 

 membrane, the membrana tectoria. The common cavity between the walls 

 of the osseous and membranous labyrinth in the vestibule, the semi- 

 circular canals, in the scala vestibuli and scala tympani of the cochlea, is 

 filled with a clear fluid the perilymph; the common cavity within the walls 

 of the entire membranous labyrinth is also filled with a similar fluid the 

 endolymph. The hair-like processes of the epithelial cells covering the 

 maculae acusticae and the rods of Corti are consequently bathed by endo- 

 lymph. Both fluids are in relation with the subarachnoid lymph-spaces at 

 the base of the brain, the perilymph through the aqueduct of the vestibule, 

 the endolymph through the endolymphatic duct. 



The Cochlear Nerve. The fibers of the cochlear nerve, arising from the 

 ganglion cells of the spiral ganglion situated in the osseous lamina spiralis 

 near the modiolus, send their peripheral branches to the saccule and to the 

 organ of Corti. As they approach this structure they lose their medullary 

 sheath and become naked axis-cylinders. The fibers then divide into two 

 parts, of which one passes to the inner hair cells; the other passes between 

 the inner rods and crosses the tunnel between the outer rods to the outer hair 

 cells. The exact method of termination of these fibers in the hair cells is 

 unknown. 



From the relation of the nerve-fibers to the organ of Corti the latter must 

 be regarded as the highly specialized terminal organ of the cochlear division 

 of the auditory nerve. 



THE PHYSIOLOGY OF HEARING 



The general function of the ear is the reception of atmospheric vibrations 

 and the transmission of the effects they produce to the percipient elements 

 the hair cells of the organ of Corti. The vibratory excitation of these end- 

 organs thus caused, is the basis of auditory perceptions. The atmospheric 

 vibrations are collected by the pinna and concha, conveyed by the auditory 

 canal to the tympanic membrane, transmitted by the chain of bones to the 

 labyrinth to pass successively through the perilymph, the membranous walls, 

 and the endolymph, to the hair cells. The nerve impulses generated by these 

 vibrations are then transmitted by the cochlear (acoustic) nerve to the acous- 

 tic centers of the cerebrum, where the sensations of sound are evoked. In 

 this general process each one of the individual structures composing the ear 

 in its entirety, has a special function. In order to fully appreciate these 

 functions the characteristic features of atmospheric vibrations, viz. : intensity 

 pitch and quality must be kept in mind. 



Atmospheric Vibrations. The vibrations of the atmosphere, the ob- 

 jective causes of the sensations of sound, are imparted to the atmosphere by, 

 the vibrations of elastic bodies such as tuning-forks, rods, strings, membranes, 

 etc These produce in the air a to-and-fro movement of its particles, r 

 suiting in a succession of alternate condensations and rarefactions which are 

 propagated in all directions. The impact of a rhythmic succession of such 

 condensations on the ear gives rise to musical sounds; the impact of an 

 arrhythmic or irregular succession gives rise to noises. 



