532 



ORGAN OF HEARING. 



the internal wall of the tube of the cochlea ne- 

 cessarily forms where it bends abruptly ut the 

 last turn. This last part of the axis, viewed 

 from the cavity of the second turn of the tube, 

 has a funnel-like appearance, the wide mouth 

 corresponding to the cupola; hence it is called 

 infundibulum or scyphus. But viewed from the 

 last turn, the so-called infundibulum is a mere 

 free edge which proceeds directly to be con- 

 founded with the walls of the cochlea. But 

 all this, if the disposition of a snail's shell, or 

 a tube coiled round be rightly conceived, is 

 understood of itself. 



Exposed by the removal of the outer walls 

 of the cochlea, the axis is somewhat like the 

 common pictorial representations of the tower 

 of Babel. It has a spongy porous appearance. 

 It is pervaded by numerous small canals which 

 run from its base onwards to orifices on its 

 sides, corresponding to the spiral lamina, and 

 transmit into the cochlea the ramifications of 

 the cochlear branch of the auditory nerve and 

 bloodvessels. The outermost of the canals are 

 the shortest; towards the interior they gradually 

 become longer, and there is one canal in parti- 

 cular wider than the rest, which runs through- 

 out its whole length ; it is called tubulus cen- 

 tralis modioli, and opens at the so-called in- 

 fundibulum. 



The base of the axis corresponds to the an- 

 terior part of the inferior depression at the 

 bottom of the internal auditory meatus, and 

 presents the commencing orifices of the small 

 canals just mentioned, arranged in a spiral 

 manner corresponding to the turns of the coch- 

 lea, tructus spiralis foraminulentus of Co- 

 in gno.* 



Spiral lamina and scala of the cochlea. 

 The passages into which the tube of the coch- 

 lea is subdivided are called scala, and the par- 

 tition lamina spiralis. 



The spiral lamina is partly bony, partly 

 membraneous; but as we are describing the 

 osseous shell of the labyrinth only, it is with 

 the bony part alone we have at present to do. 

 The bony part of the spiral lamina, zonula ossea 

 lamina spiralis, is coiled round the axis or cen- 

 tral pillar of the cochlea like the stairs in a 

 spiral staircase. The internal or central margin 

 of the bony spiral lamina is inserted on the 

 axis. Its peripheral margin is free in the dry 

 bone, so that the two scalae are not found com- 

 pletely separated from each other, as in the re- 

 cent state, when the membraneous extension of 

 the spiral lamina exists. At the place where 

 the spiral lamina is inserted on the axis, there 

 is a sort of canal all round, which has been 

 specially described by Rosenthalf under the 

 name of canalis spiro.lis modioli. 



The spiral lamina commences with a bend 

 or sweep upwards and forwards at the base of 

 the cochlea, below the hemispherical depres- 

 sion of the vestibule and opposite the bridge of 

 bone which separates the vestibular fenestra 



* De aquediictibus auris humanae internae ana- 

 tomica dissertatio, s. xxiv. pp. 3638. Viennae, 

 1774. 



t Ueber den Ban der Spindel im menschlichen 

 Ohr. In Meckel's Archiv. Bd. viii. p. 75. 



from the cochlear fenestra. Its broadest part, 

 which is about the middle of the first turn of 

 the cochlea, is about one-twentieth of an inch. 

 Towards the summit of the cochlea it insen- 

 sibly contracts, and ceasing to be connected 

 to the axis, where the latter presents the free 

 margin already mentioned, terminates at the 

 commencement of the third turn in a curved 

 hook-like point. This hook, hamulus lamina 

 spiralis, has a free concave margin towards the 

 axis, and a convex margin, which latter, how- 

 ever, like the rest of the peripheral margin of 

 the bony spiral lamina, is not free in the recent 

 state, but is continuous with the membrane 

 which completes the partition. 



In consequence of the above mode of termi- 

 nation of the bony spiral lamina by means of 

 a free margin towards the axis of the cochlea, 

 an opening of communication is left, even in 

 the recent state, between the two scalae of the 

 cochlea. For this opening, which was called 

 by Cassebohm* canalis scalarum community we 

 adopt from Breschetf the name helicotrema.l 



The bony spiral lamina consists of two thin 

 plates of bone, between which run numerous 

 small canals from the central margin of the 

 lamina to its peripheral the continuation of 

 those already described in the axis, and which 

 therefore bend at a right angle in passing from 

 the axis into the spiral lamina. At the free 

 edge of the osseous part of the spiral lamina, 

 the two plates of bone are intimately incorpo- 

 rated. This part of the bony spiral lamina, 

 which is more delicate, denser, whiter, more 

 transparent, and, in the recent state, more 

 elastic than the rest, is what Breschet calls the 

 middle zone. The surface of the spiral lamina 

 corresponding to the tympanic scala is much 

 marked with striae running from the inner 

 margin to the outer. The surface correspond- 

 ing to the vestibular scala is less striated. 



Of the two scalae of the cochlea, one, scala 

 tympani, communicates with the cavity of the 

 tympanum through the fenestra rotunda or 

 cochlear fenestra, which however, in the recent 

 state, is closed by a membrane; the other, 

 scala vestibuli, opens by an oval orifice freely 

 into the vestibule, and it is only by means of 

 the communication which the tympanic scala 

 has with the vestibular scala through the heli- 

 cotrema that the former communicates with the 

 rest of the labyrinthic cavity. The tympanic 

 scala is wider at the commencement than the 

 vestibular, which on its part again is larger 

 toward the termination. Near the feneslra ro- 

 tunda there is in the tympanic scala a very 

 minute orifice, that of the aqueduct of the 

 cochlea. We shall return to the spiral lamina, 

 the scalae of the cochlea, and the mechanism of 

 the helicotrema, when speaking of the mem- 

 brane lining the labyrinthic cavity. 



The aqueducts. What are called the aque- 

 ducts are two canals of very minute calibre, 

 opening by one extremity in the labyrinthic 



* Tractatus quintus anat. de aure humana, etc. 

 Hal* Magd. 1735, s. 194, p. 12. 

 t Op. cit. s. xiv. 

 t E?u, f Xirot, where, and Tp/na, foramen. 



