THE SENSES 



959 



anatomy shows us that the cochlea is the most highly-developed 

 portion of the internal ear, the last to appear in its evolution, and 

 the most specialized. It is absent in fishes, which possess only a 

 vestibule and one to three semicircular canals. It first acquires 

 importance in reptiles, but attains its highest development in 

 mammals ; and there is every reason to believe that it is the terminal 

 apparatus of the sense of hearing. 



n 





FIG. 424. ORGAN OF CORTI (BARKER, AFTER RETZIUS). 



mb, basilar membrane ; tb, its tympanal covering ; vs, bloodvessel (vas spirale) ; 

 re, medullated distal processes of bipolar nerve-cells in the?jganglion 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, b ; p', outer pillar with its basal cell, b' ; T, 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 ; t, 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, n 1 , to n 4 , 

 cross-sections of the spiral strand of cochlear nerve-fibres. 



Functions of the Auditory Ossicles. The anatomical 

 arrangements of the middle ear suggest that the tympanic mem- 

 brane and the chain of ossicles have the function of transmitting 

 the sound-waves to the liquids of the labyrinth ; and observa- 

 tion and experiment 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 microscope. 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, consti- 



