S.34 



ORGAN OF HEARING. 



dimenUiry state. It is in fact a mere pouch 

 or diverticulum not at all coiled up, in which, 

 however, can be distinguished a part corres- 

 ponding to a lamina spiralis, which is repre- 

 sented by a cartilage, and a vestibular and a 

 tympanic scala, together with a cochlear fe- 

 nestra. This analogy, much insisted on by 

 Breschet,* I gave a brief notice of some years 

 ago.f 



The cochlea is richly supplied with nerves. 

 The spiral lamina is that part of it on which 

 its nerves expand ; this must therefore be con- 

 sidered as forming a very essential element of 

 the cochlea, and may be viewed as being in 

 the economy of that part of the internal ear 

 what the apparatus of the membraneous laby- 

 rinth is to the vestibule and semicircular 

 canals. 



The bony spiral lamina is rendered a com- 

 plete partition between the scalse of the cochlea 

 by a membraneous continuation, zonula mem- 

 brunacea lamina' spiralis s. zona Vulsalvx, 

 formed by the application against each other 

 of the membranes, which line the interior of 

 the two scalae, at the moment they are reflected 

 from the free edge of the bony spiral lamina 

 to the outer walls of the cochlea. Hence the 

 spiral partition of the cochlea, when complete, 

 is osseous at its inner or central part, and mem- 

 braneous at its outer or peripheral. 



The outer part of the osseous zone of the 

 spiral lamina is thinner than the rest; it is 

 semi-osseous, semi-membraneous, and the 

 membraneous spiral lamina at its junction 

 with it presents a fine cartilaginous stripe ; 

 hence Comparetti and Sommerring described 

 the spiral lamina as composed of concentric 

 bands or zones. They admitted four, viz. 1, 

 the inner thick part of the bony spiral lamina ; 

 2, the outer thin part; 3, the cartilaginous 

 stripe commencing the membraneous spiral 

 lamina; and 4, the rest of the membraneous 

 spiral lamina, or the membraneous spiral la- 

 mina properly so called. The first zone is con- 

 tinued into the hamulus cochleae, the second 

 ceases towards the second turn of the cochlea, 

 and the third and fourth are continued beyond 

 the hamulus cochleae, forming of themselves 

 the spiral partition in the last turn. 



It is sufficient to admit, with Breschet,| only 

 three zones ; an osseous zone, a middle zone, 

 and a membraneous zone ; the third and 

 fourth zones of Comparetti being compre- 

 hended under the latter. 



The osseous zone of the spiral lamina we 

 have already described, and alluded to the 

 middle zone. The latter, when it still exists 

 in the dry bone, appears merely as the outer 

 margin of the former. It is the narrowest of 

 the three zones, and is most distinct in the first 



* Op. cit. and also Recherchcs Anatomiques et 

 Physiologiques sur 1'organe de 1'audition chez les 

 Oiseaux. Paris, 1836. 



t " Note on the ear of Birds," in the first and 

 only volume of the second scries of the Edinburgh 

 Journal of Natural and Geographical Science. 

 Edinburgh, 131. 



J Op, cit. chap. ix. 6. cxcix. 



Fig. 234. 



The axis of the cochlea and spiral lamina isolated, 

 in order to show the disposition of the three zones. 

 The vestibular lamina of the usseous xone is re- 

 moved. (From Breschet.) 



A, natural size. B, magnified. 

 a. trunk of the cochlear nerve *, b. distribution of 

 the filaments of this nerve in the osseous zone ; 

 c. nervous anastomoses in the middle zone; rf. 

 membraneous zone ; e. osseous substance of the 

 axis ; f. helicotrema or hole of communication 

 betwixl the two scalae. 



turn of the cochlea. Breschet describes it as 

 composed of the membranes lining the interior 

 of the two scalae, where they first meet each 

 other in passing from the bony spiral lamina, 

 together with osseous particles deposited be- 

 tween them. In this interstice between the 

 membranes also are contained the last rami- 

 fications of the filaments of the cochlear nerve, 

 still enveloped by their neurilemma, and 

 sprinkled over by the small bony particles just 

 mentioned. 



Different from the middle zone, the mem- 

 braneous zone goes on increasing in breadth, 

 though not regularly, from the base to the 

 summit of the cochlea. It is the longest and 

 most extensive of the three zones. It is it 

 alone which extends into the last turn of the 

 cochlea. According to Breschet the mem- 

 braneous zone should be composed of three 

 layers, the two exterior of which should be, 

 as already said, formed by the membranes 

 lining the interior of the scalse, and the mid- 

 dle one by the expansion and interlacing of the 

 neurilemmatic sheaths from the middle zone ; 

 but these layers are so thin and so closely 

 united that they are inseparable, and constitute 

 a membrane of great thinness and transparency, 

 on which, however, bloodvessels can be easily 

 seen. 



The membraneous zone presents a central 

 margin continuous with the rest of the spiral 

 lamina, except in the third turn of the cochlea, 

 where this margin forms nearly the third of 

 the circumference of the helicotrema, and 

 where it runs into the peripheral margin at an 

 acute angle. The peripheral margin, which 

 is much thicker than the rest of the mem- 

 braneous zone, is pervaded by a vascular sinus, 

 like that which in the eye runs round the 

 circumference of the cornea at the insertion of 

 the iris. 



