102 



THE EAR. 



it winds. It is partially divided along its whole extent by a spiral lamina, 

 projecting into it from the modiolus. From this osseous spiral lamina membranous 

 structures are in the recent condition stretched across to the outer wall of the tube, 

 and thus completely separate two passages or scalw, one on each side of the spiral 

 lamina, which communicate one with the other only by a small opening, named 

 helicotrema, placed at the apex of the cochlea. 



SceUAtynpani, ScoUa. 



Fig. 111. THE COCHLEA AND VESTIBULE EXPOSED BY REMOVING THE ROOF OF THE BONY LABYRINTH 



WITH THE SAW, SO AS TO VIEW THE PARTS FROM ABOVE. Magnified. (Testllt. ) 



That the cochlea is justly to be considered as an elongated tube, coiled spirally on the 

 modiolus, is illustrated by the simple pouch-like form of the rudimentary cochlea of birds 

 (fig. 112) as well as by the history of its development. 



The spiral osseous canal is about 33mm. long, and about 2mm. in diameter at 

 the commencement, where it is widest. From this point the canal makes nearly 2J 

 turns round the central pillar (from left* to right in the right ear, and in the opposite 

 direction in the left ear, supposing the cochlea viewed from the base), and ends by 

 an arched and closed extremity called the cupola, which forms the summit of the 



Fig. 112. OSSEOUS LABYRINTH OF THE BARN OWL (STRIX FLAMMEA). (From 



Breschet.) 



1, semicircular canals ; 2, vestibule ; 3, cochlea in the form of a short straight 

 tube. 



cochlea. The first coil, having by far the most extensive curve 

 and being the largest portion of the tube, nearly hides the second 

 from view ; and, bulging somewhat into the tympanum, forms 

 the round elevation on the inner wall of that cavity called the 

 promontory. The last half-coil is somewhat flattened from above 

 down and its extremity is partly embedded in the coil next below it. 

 The modiolus (columella cochleae), the central pillar or axis of the cochlea, is 

 much the thickest within the first turn of the tube, rapidly diminishing in size in 

 the succeeding parts. Its central part is spongy as far as the last half-coil, and is 

 pierced by many small canals, for the passage of the nerves and vessels to the spiral 

 lamina ; one of these canals, larger than the rest, central canal of the modiolus, runs 

 from the base through the centre of the modiolus (fig. 114). The base of the 

 modiolus appears in the internal auditory meatus as the fossula cochlese containing 

 the foramen centrale and the tractus spiralis foraminulentus : the latter transmit- 



