380 AN A 31 ERICA N TEXT-BOOK OF PHYSIOLOGY. 



The researches <»t' Howard Avers' have led him to conclusions concerning 

 the minute anatomy of the ear materially different from those just presented. 

 Thus, Avers asserts that the so-called membrana tectoria is nothing more 

 than the matted mass of hairs " which spring from the tops of the hair-cells 

 and form a waving plume on the crest of the ridge of the organ of Corti." 

 He also holds the membrana reticulata and several other structures 

 described by different authors to be nothing more than artefacts produced by 

 the methods of preserving and manipulating the specimens. According to 

 Avers, the cochlear nerves end in the hair-cells and not freely between them, 

 and they are probably continuous with the auditory hairs. 



Theory of Auditory Sensation. — It can hardly be doubted that the 

 nervous structures of the cochlea form an organ of special sense for the per- 

 ception of musical times and probably of noises as well. But no trustworthy 

 conclusion can be maintained as to the precise mode of action of the auditory 

 apparatus, due fact that the rods of Corti are absent from the cochlea? of 

 birds, which evidently are capable of appreciating musical tones, shows that 

 these structures may be accessory, but are not essential parts of the sensory 

 apparatus. Starting from the fact that the basilar membrane splits readily 

 in a radial direction, in which, moreover, it is tightly stretched between its 

 attachments, Helmholtz 2 long ago proposed the theory that the basilar 

 membrane behaves toward vibrations reaching it like a series of stretched 

 strings. As the wires of a piano have different rates of vibration according 

 to their length, and respond sympathetically to correspondingly different 

 notes sounded in their neighborhood, so it has been supposed that different 

 radial fibres of the basilar membrane are set into sympathetic vibration by 

 different rates of vibration in the fluids bathing them. These vibrations 

 must be imparted to the structures in the organ of Corti, and the irritation 

 of the nerves connected with the cells of Corti is a natural sequel. It may 

 he repeated that, though the canal of the bony cochlea as a whole diminishes 

 in diameter from base to cupola, the canal of the membranous cochlea, the 

 scala media with its lower wall or basilar membrane, increases in diameter. 

 Thus the radial fibres of the basilar membrane are longest near the apex of 

 the cochela. The radial width of the basilar membrane, measured near the 

 bottom, middle, and top. respectively, is given as 0.21 millimeter. 0.3 1 milli- 

 meter, ami 0.36 millimeter. The waves of physical sound are thus supposed 

 to he analyzed in the peripheral sense-organ, each auditory nerve-fibre excit- 

 ing in consciousness a tone of a particular pitch, and the mind perceiving 

 the simultaneous effects of different pendular vibrations as notes of different 

 quality. 



'Avers: Journal of Morphology, May, 1892. 



2 Helmholtz : Toiienipjiii(/iut(/i-ii, 1*77, S. 210. 



