ORGAN OF HEARING IN MAMMALIA. 



221 



Section of Cochlea, parallel with its axis, niagn. xevi 



lining membrane extends from this plate to the outer wall, fig. 164, 



d, e, and completes the separation of the two scalar. The attach- 

 ment of the base of the ' lamina spiralis ' is not solid, but indicates 

 its constitution by two confluent layers, which here separate and 

 intercept the minute channel called ' canalis spiralis modioli.' 

 Towards the apex of the 



cochlea the spiral plate 

 becomes free, and forms 

 the part called ' hamulus/ 

 fig. 164, 7. Here the two 

 scala? intercommunicate, 

 as shown by the bristle in 

 fig. 164, which emerges 

 above at the opening 

 termed ( helicotrema,' ib. 

 8 : the apical part of the 

 spiral lamina is formed by 

 an onward extension of the 

 lining membrane of the 

 cochlea, bounding the up- 

 per part of the columellar canal called ( infundibulum,' ib. 2. In 

 the section here exhibited the lower, 5, is the tympanic, the upper, 

 c, the vestibular, division of the whorl divided by the partition, b, 



e, which is thus seen to be formed 

 by the osseous plate supporting 

 the nerve-filaments, b, the layer 

 of membrane lining the tympanic 

 scala, 5, and that lining the vesti- 

 bular scala, 6 ; the two coalescing 

 beyond the bone, and becoming 

 thickened at e, where they again 

 pass into the parietal lining. The 

 cochlea is essentially a develop- 

 ment of the petrosal capsule im- 

 mediately related to the bone of 

 the head and its vibrations. The 

 membranous labyrinth, fig. 165, 

 retains, in Mammals, its common 

 vertebrate character, extending 

 through the semicircular canals and vestibule, but not beyond the 

 part of the latter whence the cochlea is prolonged to its mam- 

 malian extent : the sacculus, ib. /, retains the homologue of the 

 otolite of that part in fishes and reptiles ; the second otolite, e, is 

 also commonly present in the body of the vestibule : both are in 



165 



Membranous 



nth, wit 

 Magnified, xevi-, 



