ANIMAL MECHANISM. 301 



gentlemen pursuing medical studies, will derive the most profit 



from this pi an. 



a to c The lamina spiralis viewed from above. The distribution 



of the nerve will not be easily distinguished I fear a a a, the 



first turn ; bb, second turn ; cdc, the third turn of the lamina ; 



d e, where the scalas communicate. 



Comparctti has described, so says Mr Abernethy, in Rees' Cy- 

 to consist of four different substances, or zones : 



omparctt as escre, 

 clopaedia, the lamina to consis 

 1, the bony zone ; 2, coriace 



y zone ; 2, coriaceus; 3, vesicula; 4, the membraneous 

 zone. 

 f, sacculus sphericus ; g, space between that and the alveus coin- 



munis; h, alveus communis ; 1 k i3, posterior canal ; 1 i, its am- 



pulla; A-, the nerve expanded over it; 2Zwi, the superior canal; 



/, the ampullula? ; 4 n 5, the exterior canal, communicating at both 



ends with the alveus communis. 



Within these bony tubes, are membraneous ones, 

 prolongations of the sacs found in the vestibule ; but 

 they are not in contact with the walls : on the contrary, they 

 are kept from them by the interposition of a fluid, whose 

 equal pressure keeps them exactly in the centre. Further 

 to show the exceedingly minute structure of this accu- 

 rately operating instrument, it is necessary to remember 

 that the membraneous tube, being already a wheel within 

 a wheel, is also , distended with a transparent watery 

 liquor. Still smaller canals, running through the petrous 

 portion of the temporal bone, in which the internal ear 

 is located, pour in and discharge the old fluid, as au un- 

 ceasing process. 



COCHLEA. 



The third and last anatomical division of the internal 

 ear, is the cochlea, or snail shell. Recollecting how 

 the canal of a snail shell winds about a central pillar, 

 will enable the reader to understand the text. In the 

 snail shell of the ear, however, there are two canals, side 

 by side, which wind twice and a half round a central 

 pillar, which is a hollow pillar and termed modiolus. 

 At the apex, the two canals open in one common cavity, 

 but a thin slip of bone caps over both openings as well as 

 over the top of the hollow end of the pillar, like a parasol. 

 This is the cupola, in technical language. The upper 

 end of the hollow pillar is broad, but becoming narrower, 

 hence it is denominated the inj'undibulum or tunnel 

 shaped extremity. Most writers on this subject describe 

 two pillars, as constituting the centre, but it is unneces^ 



