CHAP, ix.] NERVOUS SYSTEM AND ORGANS OF SENSE. 



303 



continuous with the cells bearing hair-like processes, which have 

 been described as lying beside the rods of the organ of Corti. 



Thus here again as in the organs of sight, smell, and taste the 

 ultimate structure which ministers to special sense is a system of 

 rod-like filaments proceeding from spheroidal cells. 



Apart from the special function of the ear hearing the various 

 accessory structures serve the following 

 purposes in aiding that function : the 

 pinna serves to collect the sonorous 

 waves, by its prominences and ex- 

 cavations, and to direct them towards 

 the aperture of the meatus externus. 

 This collecting process is greatly aided 

 by the muscles, which enable the pinna 

 to be turned in various directions. At 

 the bottom of the meatus, the sonorous 

 waves act upon the tympanic membrane, 

 the vibrations of which are conveyed 

 by the auditory ossicles across the tym- 

 panum to the fenestra ovalis and so to 

 the perilymph, and, finally, through 

 the walls of the membranous labyrinth, 

 to the hair-like processes projecting 

 into the endolymph of the ampullae, 

 and to those of the organ of Corti. The 

 vibrations of the rods of the last named 

 organ doubtless intensify the vibratory 

 action, as do the otoliths enclosed within 

 the utricle and sacculus. 



The Eustachian tube places the air 

 inside the tympanum in communication 

 with the exterior, and so prevents undue 

 tension. 



The stapedius and tensor tympani 

 muscles tend by their contraction to 

 tighten the tympanic membrane and 

 that of the fenestra ovalis, and so to 

 moderate the effect of too great sound. 

 Thus the labyrinth, with its fluid con- 

 tents, can be affected either by aerial 

 waves through the meatus, tympanum, 

 and fenestra ovalis, or through the bones 

 and solid structure of the head. 



27. Certain marked ANALOGIES 

 exist between the ear and the eye, with 



certain noteworthy differences. Both are protected by skull bones, 

 but the ear much more completely so. Both are protected by external 

 folds of integuments furnished with muscles the pinna and the 

 eyelids. Both are supplied by a nerve of special sense, which enters 



Fig. 140. DIAGRAM OF THE AUDI- 

 TORY EPITHELIUM AND THE MODE 

 OF TERMINATION OF THE NERVES 

 OF THE AMPULLAE. 



c. Columnar epithelium. 



sp. Spindle-shaped cells, each support- 

 ing an auditory hair, h. 



T}. Basal supporting cells. 



n. Two nerve-fibres, passing through 

 the layer of membrane (called the 

 tunica propria) immediately be- 

 neath the epithelium, to join the 

 plexus in the epithelium. 



I, I. Limit of the membrane beneath 

 the epithelium. 



