530 



ORGAN OF HEARING. 



of vestibule, semicircular canals, and cochlea. 

 The semicircular canals and cochlea do not com- 

 municate immediately with each other, but only 

 mediately through the vestibule. The latter 

 may be considered the principal compartment. 



The osseous labyrinth is imbedded in the 

 substance of the petrous portion of the temporal 

 bone, from the compact texture of which it is, 

 in the adult, scarcely to be distinguished. In 

 the early periods of life, however, its walls con- 

 sist of a hard but brittle osseous substance, 

 around which is the then less compact tissue of 

 the petrous bone. Hence it is in a young bone 

 only, and that by means of some little prepara- 

 tion, that the external form of the osseous laby- 

 rinth can be well demonstrated. 



Of the compartments of the osseous laby- 

 rinth, the vestibule lies in the middle, the semi- 

 circular canals behind it, and the cochlea in 

 front. 



Kg. 232. 



The exterior of the ossemts labyrinth of the left side. 

 Natural size. 



a. Oval or vestibular fenestra ; 6. round or 

 cochlear fenestra ; c. external or horizontal semi- 

 circular canal ; d. superior or anterior vertical se- 

 micircular canal; e. posterior or inferior vertical 

 semicircular canal ; /. the turns of the cochlea. 



The vestibule, (vestilulum ; Fr. le vestibule ; 

 Germ, der Vorhof.) The vestibule is an irre- 

 gularly shaped cavity, the diameter of which 

 from above downwards, as also from behind 

 forwards, may be stated to be about one-fifth 

 of an inch. The distance between its inner 

 and outer wall is somewhat more than one- 

 tenth of an inch. In an anatomical sense we 

 can distinguish in it three horns, one of which 

 is towards the anterior and lower part, another 

 towards the posterior and lower part, whilst 

 the third composes the upper part of the vesti- 

 bular cavity. 



The anterior and lower horn leads by an 

 oval opening directed forwards and downwards 

 into the vestibular scala of the cochlea. This 

 opening is called the vestibular orifice of the 

 cochlea, osteum s. apertura scalie vestibuli 

 cochlea. The posterior and lower horn of the 

 vestibule corresponds to three of the orifices of 

 the semicircular canals ; the upper horn to 

 the other two orifices. 



At the under part of the inner wall of the 

 vestibule, within the limits of its anterior horn 

 and to the inside of the vestibular orifice of the 

 cochlea, is a hemispherical depression, fovea 

 liemispherica s. sub-rotunda. Its bottom, which 

 corresponds to the posterior part of the lower 

 depression at the bottom of the internal audi- 

 tory meatus, presents a sieve-like spot, macula 

 cribrosa, that is, it is perforated by minute 

 apertures for the passage of filaments of the 

 auditory nerve. On the upper wall of the ves- 

 tibule, bordering the upper margin of the hemi- 



spherical fossa and within the limits of the 

 upper horn, is another depression, of an oval 

 shape, which is known by the name of fovea 

 hemi-elliptiea. The hemispherical and hemi-el- 

 liptical depressions are separated by a ridge or 

 pyramidal eminence, eminentia puramidalis, 

 pervaded hy small canals for the passage also 

 of nervous filaments. On the inner wall of the 

 vestibule, a little in front of the orifice common 

 to the two vertical semicircular canals and 

 within the limits of the posterior horn, there is, 

 bordering on the hemi-elliptical and hemisphe- 

 rical depressions, below the former and behind 

 the latter, another very small depression or 

 sulcus, fossa s. cavitas sulciformis, which leads 

 upwards and backwards to a small oblique 

 orifice, that of the aqueduct of the vestibule, 

 osteum internum aijueduclus vestibuli. At the 

 middle of the inner wall of the vestibule, where 

 the boundary lines of these three depressions 

 meet, there is a slight eminence. 



The inner wall of the vestibule corresponds 

 to the bottom of the internal auditory meatus, 

 and is pervaded by small canals, some of which 

 have been already mentioned, for the passage 

 of fibrils of the auditory nerve and of blood- 

 vessels. 



In the outer wall of the vestibule there is an 

 oval, or rather a kidney or bean-shaped hole, 

 called foramen ova/e, s. fenestra ovalis, s. fenes- 

 tra vestibuli. The long diameter of this aper- 

 ture, which is about one-tenth of an inch or 

 perhaps a little more, is directed from behind 

 forwards. Its vertical diameter is about half 

 that of its long diameter. The upper part of 

 the circumference of the hole is arched up- 

 wards, the lower part is slightly inclined in the 

 same direction. The margin of the vestibular 

 fenestra is turned in towards the vestibule. 

 Viewed from the tympanum, into which it 

 opens in the macerated bone, the vestibular 

 fenestra appears situated at the bottom of a 

 fossa, which was called by Cotugno pelvis 

 ovalis. In the recent state the vestibular fene- 

 stra is closed in by the base of the stapes. 



The semicircular canals, (canalcs semicircu- 

 larcs ; Fr. les canaux scmic irculaires ; Germ, die 

 Bogengdnge oder hulbcirkelf'orrnigen Candle.) 

 These are three canals, which, describing more 

 than the half of an irregular circle, open at each 

 of their extremities into the vestibule ; hence, 

 if it was not for the circumstance that two 

 unite by one of their extremities to form a 

 common short canal, there would be in the 

 vestibule six orifices of semicircular canals, in- 

 stead of the five 'only which exist. The calibre 

 of these canals is about one-twentieth of an inch 

 in the direction from the concavity to the con- 

 vexity of their curve; in the opposite direction 

 they are somewhat compressed, so that a trans- 

 verse section, instead of presenting a round 

 orifice, presents an elliptical one. The semicir- 

 cular canals are wider where they open into the 

 vestibule, but especially so at one of their ex- 

 tremities, which presents a dilatation in the 

 form of a bulb, called ampulla or ampullary 

 sinus, sinus ampullaceus. 



Two of the semicircular canals occupy a 

 vertical position and one of them a horizontal. 



