I030 THE SENSES 



ment fully confirm this idea. Tracings of the movements of the 

 ossicles have been obtained by attaching very small levers to them, 

 and their movements have been directly observed with the micro- 

 scope Even in man it may be shown, by viewing the membrane 

 through a series of slits in a rapidly-revolving disc (stroboscope) , 

 that it vibrates when sound-waves fall on it. 



When the handle of the malleus moves inwards, rotating around 

 an axis which may be supposed to pass through its neck, its head 

 moves in the opposite direction. The joint between that bone and 

 the incus is thus locked, on account of the shape of the articular 

 surfaces. The long process of the incus, constituting the second 



mb 



7'C 



Fig. 439. Organ of Corti (Barker, after Retzius). mb, basilar membrane; tb, its 

 tympanal covering; us, bloodvessel (vas spirale); re, medullated distal processes 

 of bipolar nerve-cells in the ganglion spirale, passing in to arborize around the 

 hair -cells; iS, epithelial cells continuous with the epithelium of the sulcus 

 spiralis internus; p, inner pillar of Corti, with its basal cell, 6 ; p', outer pillar 

 with its basal cell, 6' ; i, 2, 3, supporting cells of Deiters, whose processes run up 

 to be attached to the lamina reticularis, r ; H, Hensen's supporting cells; C, cells 

 of Claudius; i, internal hair-cell with its hairs, i' (the upper part of the hair-cell 

 is concealed by the head of the inner pillar of Corti) ; e, external hair-cell; e', hairs 

 of three external hair -cells; , n 1 , to w 4 , cross-sections of the spiral strand of 

 cochlear nerve-fibres. 



portion of the bent lever, passes inwards, carrying with it the stapes, 

 which is attached to it by art almost rigid joint, and the stapes is 

 pressed into the oval foramen. Since the long process of the incus 

 is about one-third shorter than the handle of the malleus, the ex- 

 cursion of the point of the former is correspondingly smaller than 

 that of the latter, but at the same time more powerful. When the 

 tympanic membrane passes outwards, the handle of the malleus 

 and foot of the stapes do the same. But the joint now unlocks, and 

 excessive outward movement of the stapes, which might result in 

 its being torn from its orbicular attachment, is prevented. The 

 ossicles vibrate en masse. It is only to a trifling extent that sound 



