THE IXTERXAL EAR. 923 



auditory nerve. It measure? about a quarter of an inch in length, and its breadth 

 toward the base is about the same. It consists of a conical-shaped central axis, 

 the modiohi* or cfilumella ; of a canal wound spirally round the axis for two turns 

 ind a half, from the base to the apex : and of a delicate lamina (the lamina apiralis) 

 contained within the canal, which follows its windings and partially subdivides it 

 : nto tw.. ; . 



The o.ntral axis, or //>'7"</>/>-. is conical in form, and extends from the base to 

 the apex of the cochlea. Its base is broad, corresponds with the first turn of the 

 cochlea, and is perforated by numerous orifices, which transmit filaments of the 

 cochlear branch of the auditory nerve ; the axis diminishes rapidly in size in 

 the second coil, and terminates within the last half-coil, or cupola, in an expanded 

 delicate, bony lamella, which resembles the half of a funnel divided longitudinally, 

 and is called the infundibulum ; the broad part of this funnel is directed toward 

 the summit of the cochlea, and blends with the cupola or last half-turn of the 

 spiral canal of the cochlea. At this point the two larger scalte of the cochlea, the 

 scala tympani and .seal a vestibuli, communicate by an opening called the helico- 

 tr> ///'./. The outer surface of the modiolus forms part of the wall of the spiral 

 canal, and is dense in structure; but its centre is channelled, as far as the last 

 half-coil, by numerous branching canals, which transmit nervous filaments in 

 regular succession into the canal of the cochlea or on to the surface of the 

 lamina spiralis. One of these, larger than the rest, occupies the centre of the 

 modiolus, and is named the canalis centralis modioli ; it extends from the base to 

 the extremity of the modiolus, and transmits a small nerve and artery (arteria 

 centralis modioli). 



The spiral canal (Fig. 5-48) takes two turns and a half round the modiolus. 

 It is about an inch and a half in length, measured along its outer wall, and 



FIG. 548. The cochlea laid open. (Enlarged, i 



diminishes gradually in size from the base to the summit, where it terminates in 

 a cul-de-zac. the cupola, which forms the apex of the cochlea. The commence- 

 ment of this canal is about the tenth of an inch in diameter; it diverges 

 from the modiolna toward the tympanum and vestibule and presents three open- 

 ings. One. the fenestra rotunda, communicates with the tympanum ; in the 

 recent state this aperture is closed by a membrane, the membrana tympani 



daria. Another aperture, of an oval form, enters the vestibule. The 

 third is the aperture of the aqueductus cochlea?, leading to a minute funnel-shaped 

 canal, which opens on the basilar surface of the petrous bone and transmits a 

 small vein. 



The interior of the spiral canal (Fig. 549) is partially divided into two. in the 

 dry state, by a thin bony plate, the lamina spiral!*, which consists of two thin 

 lamellae of bone, between which are numerous canals for the passage of nerve- 

 fibres. This lamina projects from the modiolus into the canal, but does not reach 

 more than halfway toward the outer wall of the tube. From its extremity a thin 

 membrane extends to the outer wall, and completes the division of the canal into 

 an upper compartment, the scala vestibuli. and a lower one. the scala tympani. 



