258 OF SENSATION, AND THE ORGANS OF THE SENSES. 



between the true Reptiles and Fishes, there is a remarkable variation in this 

 respect some having a tympanum, and some being completely destitute of 

 it. Wherever a tympanic cavity distinctly exists, there is an Eustachian tube 

 connecting it with the fauces. This cavity, in the true Reptiles, not only 

 possesses the fenestra ovalis (or opening into the vestibule) but the fenestra 

 rotunda (or opening into the cochlea). The membrana tympani is usually 

 visible externally ; but it is sometimes covered by the skin. In Birds, the 

 structure of the ear is essentially the same, as in the higher Reptiles. A dis- 

 tinct cochlea exists, though its form is not spiral but nearly straight : of its 

 character, however, there can be no doubt ; a division into two passages, by a 

 membranous partition on which the nerve is spread out, being evident. More- 

 over, the tympanum communicates with cavities in the cranial bones, which 

 are thus filled with air; and these, by increasing the extent of surface, pro- 

 duce a more powerful resonance. There is no external ear, except in a few 

 species of nocturnal Birds. In Mammalia, the organ of hearing is usually 

 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 

 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 external 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. 



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 vestibula of the first turn; 6, section of the lamina spiralis, its zonula ossea; 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 spiralis ; 8, loops formed by 

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

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

 he rema.nmg 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. 



352. The ultimate terminations of the fibres of the auditory nerve in minute 

 papillae are best seen in the lamina spiralis of the cochlea, and its membranous 

 prolongation. Much diversity exists, however, as to the interpretation of the 

 appearances there seen; some observers affirming that there are no free or 

 papillary terminations, and that the nervous fibres all return by loops; whilst 



