ORGAN OF HEARING. 



533 



cavity, and by the other on the surface of the 

 petrous portion of the temporal bone. They 

 are generally associated with the name of Co- 

 tugno,* who, though not their discoverer, was 

 the first to give a complete description of them. 

 One, called aqutductus vtstibuli, communicates 

 with the vestibule ; the other, aqueductus coch- 

 letf, with the tympanic scala of the cochlea. 



The internal orifice of the aqueduct of the 

 vestibule is observed to commence by a groove 

 or sulcus, the sulciform depression already de- 

 scribed in the vestibule, immediately below 

 and in front of the opening common to the two 

 vertical semicircular canals. From this the 

 aqueduct turns itself round the inner wall of 

 the common canal, and then follows a course 

 downwards and backwards. Gradually widen- 

 ing, it opens under that sort of osseous scale 

 observed a little behind the middle of the pos- 

 terior and inner surface of the petrous bone, 

 just above the jugular fossa ; towards the latter 

 there is usually a groove running on the surface 

 of the bone from the orifice of the aqueduct. 

 The length of the course of the aqueduct of 

 the vestibule is about one-third of an inch. 



The aqueduct of the cochlea commences by 

 a very small orifice in the lower wall of the 

 scala tympani immediately before the fenestra 

 rotunda. It proceeds downwards, inwards, 

 and forwards, in the inner wall of the jugular 

 fossa of the temporal bone, and widening in 

 its course it opens at the bottom of that tri- 

 angular pyramidal depression, situated towards 

 the middle of the edge which limits the inner 

 and inferior surfaces of the petrous bone, and 

 below the internal auditory meatus. The 

 length of its course is about a quarter of an inch. 

 The aqueduct of the cochlea is very wide in 

 the pig. Of the aqueducts we shall observe 

 farther in speaking of the membrane lining 

 the labyrinthic cavity. 



Fig. 233. 



The labyrinthic cavity of the riyht side, magnified 

 two diameters. 



a. superior horn of the vestibule ; b. posterior 

 and inferior horn ; c. anterior and inferior horn 

 leading into the cochlea ; rf. hemispherical depres- 

 sion ; e. hemi-elliptical depression ; /. pyramidal 

 elevation between the two having a porous sieve - 

 like appearance from being pervaded by canals for 

 the passage of nervous filaments ; g. superior 

 vertical semicircular canal; h. its ampullary dila- 

 tation ; t. posterior vertical semicircular canal; 

 k. its ampullary dilatation; /. canal common to 



the superior and posterior vertical semicircular 

 canals ; m. orifice by which the common canal 

 opens into the vestibule ; n. horizontal semi- 

 circular canal; o. its ampullary dilatation; p. 

 vestibular orifice of the aqueduct of the vestibule ; 

 q. osseous part of the spiral lamina, seen from the 

 surface which corresponds to the vestibular scala ; 

 r r. space which is occupied by the membraneous 

 part of the spiral lamina ; *. hamulus or hook in 

 which the bony spiral lamina ends ; t. helico- 

 trema; u. substance of the petrous bone, between 

 the first turns of the cochlea ; p. orifice of the aque- 

 ductus cochleae. 



lining the labyrinthic cavity. 

 The cavities of the osseous labyrinth which we 

 have just described are lined by a serous or 

 fibro-serous membrane, extremely delicate and 

 closely adherent to the surfaces. The mem- 

 braneous labyrinth must not be confounded with 

 it. This membrane, which may be compared 

 to that serous pellicle on the inner surface of 

 the sclerotica, known by the name of meni- 

 brana fusca or arachnoidea oculi, is more 

 manifest at an early age than in adults, and is 

 nowhere so distinct as at the places where the 

 nerves enter, and at the bottom of the tym- 

 panic scala of the cochlea. It is it which 

 completes the spiral septum of the cochlea, by 

 an arrangement immediately to be described. 



The fenestra rotunda or cochlear fenestra is, 

 in the recent state, closed by a membrane 

 which shuts out the cavity of the tympanum 

 from any direct communication with the 

 cochlea. This membrane, called by Scarpa* 

 the secondary membrane of the tympanum, 

 membrana tympani st'cundaria, is concave to- 

 wards the cavity of the tympanum, convex 

 towards the tympanic scala of the cochlea, and 

 is received at its circumference into a groove 

 within the orifice of the fenestra rotunda. It 

 is composed theoretically of three layers, the 

 inner of which is nothing but the fibro-serous 

 membrane under consideration. The outer 

 layer is a continuation of that which lines the 

 cavity of the tympanum. The third and pro- 

 per layer is situated between the two men- 

 tioned. The same may be said in re-^ard to 

 that membrane, which, together with the base 

 of the stapes, closes the vestibular fenestra. 



The membrane lining the tympanic scala 

 of the cochlea is continued into that lining the 

 vestibular scala at the opening called heli- 

 cotrema. The membrane of the vestibular 

 scala is continuous with that lining the vesti- 

 bule, which on its part is continuous with that 

 of the semicircular canals. Lastly, the same 

 membrane lines the aqueducts. 



Such is a general description of the mem- 

 brane lining the labyrinthic cavity ; but to 

 understand the disposition of the cochlea and 

 aqueducts in the recent state, we must take a 

 nearer view of this membrane such as it exists 

 in those cavities, which, indeed, is the most 

 important and difficult part of it. 



Of the cochlea in the recent state. The 

 cochlea is the last addition made to the laby- 

 rinth in the ascending scale of the animal 

 series. As was said, it is in birds in a very ru- 



Op. cit. 



* De auditu et olfacm, cap. ii. s. 19. p. 35, 



