426 



ON SENSATION, AND THE ORGANS OF THE SENSES. 



formed upon the same plan, as it presents in Man ; in the Monotremata, how- 

 ever, it more approaches that of Birds. The cochlea of the Mammalia in 

 general is a spiral, forming about two turns and a half; the partition which 



[Fig. 184. 



The Cochlea divided parallel with its axis, through the centre of the Modiolus ; after Breschet; 1, the 

 modiolus; 2, the infundibulum in which the modiolus terminates ; 3, 3, the cochlear nerve, sending its fila- 

 ments through the centre of the modiolus ; 4, 4, the scala tympani of the first turn of the cochlea; 5, 5, the 

 scala vestibulaof the first turn; 6, section of the lamina spiralis, its zonulaossea; one of the filaments of 

 the cochlear nerve is seen passing between the two layers of the lamina spiralis to be distributed upon the 

 membrane which invests the lamina; 7, the membranous portion of the lamina spiral!*; S, loops formed by 

 the filaments of the cochlear nerve; 9, 9, scala tympani of the second turn of the cochlea; 10, 10, scala 

 vestibula of the second turn ; the septum between the two is the lamina spiralis ; 11, the scala tympani of 

 the remaining half turn ; 12, the remaining half turn of the scala vestibula; the dome placed over this half 

 turn is the cupola: 13, the lamina of bone which forms the floor of the scala vestibula curving spirally 

 round to constitute the infundibulum (2) ; 14, the helicotrema through which a bristle is passed ; its lower 

 extremity issues from the scala tympani of the middle turn of the cochlea.] 



divides its canal is partly osseous, partly membranous; and its two passages 

 communicate with the tympanic cavity and the vestibule respectively. The 

 cavity of the tympanum is very large in some species, extending even into the 

 contiguous bones. All the Mammalia, except the aquatic tribes, have an ex- 

 ternal ear ; and this is sometimes of an enormous size in proportion to the 

 dimensions of the body, as it is in the Bats. The labyrinth of the higher 

 Vertebrata contains no otolithes. 



559. The ultimate terminations of the fibres of the Auditory nerve, are 



Fig. 1S5. 



[Fig. 186. 



Papilla; (?) of the Auditory nerve, 

 on a segment of the spiral lamina of 

 the cochlea of a young Mouse ; the 

 lower portion is the osseous, and the 

 higher the membranous part of the 

 lamina. Magnified 300 times. 



The Auditory Nerve taken out of the Cochlea; 1, 1, 1, 

 the trunk of the nerve; 2, 2. its filaments in the zona ossea 

 of the lamina spiralis; 3. 3. its anastomoses in the zona 

 vesicularis.] 



