OF HEADING. 



75 



minate in a common orifice, there are only five openings from 

 them into the vestibule. Fig. 54 exhibits a section of the 

 semicircular canals. 



Fig. 54. Section of 

 Canals. 



Fig. 55. Views of the Cochlea. 



Fig. 56. Anterior in- 



Anterior internal 



Surface. Base ' A P ex - 



[ 154. The cochlea (fig. 51, c, and 55) is a singular organ, in 

 form very like the shell of a garden snail. Its cavity (fig. 56) is 

 divided by a longitudinal partition, half os- 

 seous and half membranous, called the spi- 

 ral lamina (fig. 57, 29), which makes two 

 and a-half turns round a central pillar, the 

 modiolus (fig. 58, 26), the apex of which 

 is called the cupola (28). One of these 

 passages (fig. 57, 33) leads to the fora- 

 men ovale (22), of the vestibule, and is ternal Surface "of 'spiral 

 called scala vestibuli; the other (32) ter- tube; the lamina spiralis 

 minates in the foramen rotundum of the removed, 

 tympanum, and is called scala tympani. 

 These passages are freely perforated, to 

 give transit to filaments of the auditory 

 nerve, which enters the cochlea through 

 the cribriform base of the central pillar 

 (fig. 58, 35). The whole of the internal 

 ear is filled with a limpid fluid, perilymph y 



in which the membranous and nervous ralis \ the external shell 

 parts of the semicircular canals arid coch- of the cochlea removed, 

 lea are suspended. This membranous labyrinth contains a 

 similar fluid, the endolymph* T. W.] 



155. By this mechanism, the vibrations of the air are 

 first collected by the external ear, whence they are conveyed 

 along the auditory passage, at the bottom of which is the tym- 



* The figures of the internal ear, the last excepted, are copied from 

 Soemmering. 



Fig. 57. Lamina spi- 



