536 



ORGAN OF HEARING. 



Farther observations on the aqueducts. The 

 aqueducts, the one leading from the vesti- 

 bule, and the other from the tympanic scala of 

 the cochlea, are lined by a continuation of the 

 thin and delicate pellicle which invests the in- 

 terior of those cavities. 



Under the osseous scale, on the surface of 

 the petrous bone, where the aqueduct of the 

 vestibule ends, there is a small triangular pouch 

 produced by a separation of the dura mater into 

 two layers. Into this pouch the lining mem- 

 brane of the aqueduct enters, and ends in a 

 cul-de-sac. The pouch is called by Cotugno 

 the membraneous cavity of the aqueduct. I 

 found the structure just described of unusual 

 size, in consequence of irregular development, 

 in the ear of a man deaf and dumb from birth, 

 which I examined some years ago. The trian- 

 gular pouch in the dura mater was about one- 

 third of an inch long at its sides, and was dis- 

 tended by a clear liquid. Every time pressure 

 was made on the distended pouch, a fine jet of 

 liquid issued through a small opening which 

 had been made in the superior vertical semicir- 

 cular canal. Similar cases have been described 

 by Mondini* and others. 



The lining membrane of the aqueduct of the 

 cochlea ends, in like manner, in a cul-de-sac, 

 which, however, is not so large as that of the 

 aqueduct of the vestibule. 



The liquid contained in the labyrinthic ca- 

 vity, or liquid of Cotugno, or perilymph. 

 (Aquula Cotunnii.) The cavities of the 

 osseous labyrinth contain a liquid, the secre- 

 tion, probably, of their thin and delicate 

 lining membrane. They contain no air, 

 us has been asserted. The liquid called the 

 liquid of Cotugno, or by De Blainville peri- 

 lymph, must not be confounded with ano- 

 ther which is contained in the interior of the 

 membraneous labyrinth. Dominico Cotugno, -f 

 though not actually the discoverer of this liquid, 

 yet took a more correct view of it than his pre- 

 decessors in this branch of anatomical research, 

 Valsalva.J Vieussens, Cassebohm,|| and Mor- 

 gagni.H He in fact recognised in it a substance 

 fulfilling some office in the exercise of hearing, 

 a view of the matter which was admitted by 

 Haller, and put beyond doubt by Ph. Fr. 

 Meckel,** and since their time recognised by 

 all physiologists. 



The perilymph occupies, in the vestibule and 

 semicircular canals, all the space not taken up 

 by the membraneous labyrinth. The cochlea 

 contains nothing but it ; and as all the cavi- 



* Comment. Ilonon. torn. vii. Anatomia Surdi 

 Nati. p. 422. 



i ille, p. 



75. Toulouse, 1714. 



j[ Tractatus Quintus Anatomicus de Aure Hu- 



ana, &c., pp. 20-21. DC Labyrimho. H;il;r Magd. 



86. 



^| Epist. Anatorn. xii. s. b'4, p. 469. Venetiis 

 1740. 



ma 



1786. 



1740. 



** Dissertatio Anatomico-Physiologica de Labv- 

 rinthi Auris Contends, &c. Argentorati, 1777, s. 8, 



ties of the osseous labyrinth communicate, it is 

 the same humour in each. 



In all fishes.except the cartilaginous with fixed 

 gills, the labyrinthic cavity is very imperfect, 

 being in many of them open towards the cranial 

 cavity, or at the most separated from it only by 

 a membraneous partition, as in the cod; hence 

 the encephalic liquid in some of them is not dis- 

 tinct from the perilymph whose function it must 

 perform. In the cartilaginous fishes with fixed 

 gills, on the contrary, the labyrinthic cavity is 

 completely separated from the cranial ; therefore 

 in them we meet with perilymph distinct from 

 the encephalic liquid, and that, too, in pretty 

 large quantity. 



In birds the perilymph is in much less quan- 

 tity than in the mammifera, in proportion to the 

 size of the membraneous labyrinth. In rep- 

 tiles the quantity of perilymph is still less. 



Cotugno and Meckel supposed that the 

 aqueducts were a sort of diverticula, or cavi- 

 ties which served to let off the superabundant 

 perilymph, when necessary, during the act of 

 hearing. This opinion is, however, now-a-days 

 very much questioned, and several anatomists, 

 Brugnone, Kibes, Breschet, &c., refuse to those 

 aqueducts the uses which Cotugno assigned 

 them, and consider them merely as canals des- 

 tined lor the passage of bloodvessels. Although 

 they may be insignificant in a physiological 

 point of view, still, if the description 1 have 

 given of them be correct, they must be consi- 

 dered as something more than mere canals for 

 the transmission of vessels. The constancy of 

 the aqueducts, moreover, is another argument 

 against their being mere vascular canals. 



Breschet,* and in Hildebrandt's Anutomieby 

 Weberf the same idea is concisely expressed, 

 explains the mode of formation of the aque- 

 ducts by supposing that at first the labyrinthic 

 cavity is nothing but a sac formed by a prolon- 

 gation of the dura mater in the same way as the 

 tunica vaginalis is of the peritoneum ; that as 

 development proceeds, the tube of communi- 

 cation between the labyrinthic sac of the dura 

 mater and general cavity of the dura mater is 

 gradually contracted and elongated ; and that as 

 ossification extends, the tube becomes sur- 

 rounded by osseous substance, and presents 

 itself under the appearance of an aqueduct. 

 "This view," says Breschet, "is rendered 

 probable, for in many fishes the labyrinthic 

 cavity forms one with that of the cranium, 

 and if, in these animals, a prolongation of the 

 walls of the cranium tended to separate the 

 brain from the ear, there would result a small 

 canal establishing a communication between 

 the two cavities, and this canal would be 

 nothing but an aqueduct." 



According to this view, the lining membrane 

 of the labyrinthic cavity may be considered as 

 a continuation of the arachnoideal layer of the 

 dura mater, perhaps of the dura mater also. 



2. The membraneous labyrinth, ( labi/rinthus 

 mcmbranaceus. Fr. Labyrinthe niembraneii.r. 

 Germ. Das liaut/ge Labyrinth.) Within the 



* Op. cit. 



t Hand iv. p. 32. 



