544 



ORGAN OF HEARING. 



aqueduct of Fallopius, and opens on the upper 

 surface of the petrous portion of the temporal 

 bone outside and in front of the hiatus of Fal- 

 lopius. This canal, first accurately described 

 by Arnold,* and called by him the tympanic 

 canal, canalis tyrnpanicus, is traversed by the 

 nerve of Jacobson, which establishes a com- 

 munication betwixt the glosso-pharyngeal and 

 the otic ganglion. Besides this groove there 

 are several others corresponding to the branches 

 of the tympanic plexus of nerves. 



The opening below and behind the promon- 

 tory, the fenestra rotunda or cochlearj'enestra, 

 leads, by a short infundibuliform canal directed 

 obliquely inwards, into the lower or tympanic 

 scala of the cochlea. Looking into this very 

 short canal sideways, a groove is remarked en- 

 circling the margin of its inner orifice. This 

 groove receives the circumference of the secon- 

 dary membrane of the tympanum. 



The opening above the promontory, the fe- 

 nestra ovalis or vestibular j'enestra, has already 

 been described in speaking of the vestibule. 

 All that we have to add here is that it is sur- 

 rounded externally close to its edge by a small 

 channel or groove. 



Above the vestibular fenestra and running in 

 much the same direction as its long diameter 

 is a round elongated ridge, within which is the 

 aqueduct of Fallopius. Below this ridge and 

 behind the vestibular fenestra is a small mam- 

 millary or pyramidal eminence, called the pyra- 

 mid, eminent ia papillaris s. protuberantia py- 

 ramidalis. The apex of the pyramid, directed 

 forwards and a little outwards, presents an 

 opening leading into a canal, which extends 

 backwards and downwards, then becoming 

 vertical lies in front of the lower part of the 

 aqueduct of Fallopius. In the thin lamina of 

 bone which separates the two canals there is 

 an aperture. The muscle of the stapes is 

 lodged in the canal, and its tendon issues by 

 the aperture in the apex, of the pyramid. 

 About one-sixth of an inch behind the pyra- 

 mid and close to the groove for the insertion of 

 the circumference of the membrana tympani 

 is the opening by which the chorda tympani, 

 accompanied by an artery, enters the tym- 

 panum. 



In front and a little above the vestibular 

 fenestra, and on the anterior extremity of the 

 prominence of the aqueduct of Fallopius, is a 

 tubular projection with a wide open mouth 

 directed outwards. This tubular projection, 

 which is generally found incomplete in the 

 dry bone, in consequence of being composed 

 of a very thin brittle substance, is what has 

 been called the cochleariform process. It is 

 the continuation, bent at nearly a right angle 

 outwards, of the canal or half canal, about half 

 an inch in length, and destined for the recep- 

 tion of the internal muscle of the malleus, 

 which lies above the osseous part of the Eusta- 

 chian tube, and is separated from it merely by 



* Ueber den Canalis tympanicus und mastoideus, 

 in Tiedemann's, Treviranus, und Gmelin's Zeit- 

 schrift fur die Physiologic, B. iv. Heft 2, No. xxi. 

 p. 284. 



a thin lamina of bone, the continuation 

 that forming the tubular projection. 



Fig. 248. 



The inner wall of the tympanum. 



a. Promontory ; b. vestibular fenestra ; c. coch- 

 lear fenestra ; d. pyramid ; e. eminence of the 

 aqueduct of Fallopius ; /. cochleariform process 

 and half canal for the internal muscle of the mal- 

 leus. 



The outer wall of the tympanum is formed 

 by the membrana tympani and the inner ex- 

 tremity of the osseous part of the external au- 

 ditory passage, in which the membrana tympani 

 is framed. 



Osseous portion of the auditory passage. 

 This leads from the outside of the temporal 

 bone. In front of it lies the glenoid cavity, 

 and behind it is the mastoid process. It is 

 about three-quarters of an inch long. Its 

 course is from without inwards and from 

 behind forwards, at first a little upwards and 

 then downwards. It is wider at either extre- 

 mity than in its middle. A cross section of 

 the passage presents an elliptical orifice, the 

 long diameter of which is directed from behind 

 forwards and from below upwards. Its extre- 

 mities are cut obliquely in such a way that in- 

 ternally the anterior wall exceeds the posterior, 

 whereas at the outer orifice the posterior wall 

 exceeds the anterior in length. 



The margin of the outer orifice is rough and 

 irregular to give attachment to the cartilaginous 

 portion of the passage and to the auricle. Just 

 within the inner orifice the osseous auditory 

 passage is grooved all round except at its upper 

 part. This groove is for the reception of the 

 circumference of the membrana tympani. 



In the foetus the osseous portion of the audi- 

 tory passage is a mere ring of bone, the tym- 

 panic ring, incomplete at the upper part where 

 the groove in the adult is wanting. The tym- 

 panic ring serves as a frame for the membrana 

 tympani. On the inner surface of the superior 

 extremity of the anterior crus of this incomplete 

 ring of bone there is a broad superficial groove, 

 into which the processus gracilis of the malleus 

 is received. 



By-and-bye the tympanic ring is united to 

 the temporal bone, and in process of time the 

 part outside the groove grows outwards so as to 

 form that plate of bone, thick behind, thin in 

 front, rolled together in the form of an incom- 

 plete tube, which in the adult composes the 

 lower, the anterior, and the posterior walls of 

 the osseous auditory passage. 



