558 



ORGAN OF HEARING. 



distinct membranes, the one simply inclosed 

 within the other, but not connected farther. 

 The inner membrane is thinner but firmer and 

 more elastic than the outer. The latter, which 

 does not adhere to the cartilaginous case any 

 more than it does to the osseous labyrinth 

 which succeeds it, gradually becomes thin, 

 until at the seventh month there is no longer 

 any trace of it. The inner membrane, on the con- 

 trary, becomes proportionally thicker and firmer. 



Meckel has never found the membraneous 

 labyrinth in a more simple form, nor has he 

 been able to determine whether it ever exists 

 naked in the cranium. 



At a very early period the pulpy mass of 

 the auditory nerve becomes converted into 

 nervous bundles, and grows either by lateral 

 additions or by an increase of its filaments. 

 The cochlear part of it, according to Valentin,* 

 lies free in the tube of the cochlea under the 

 form of a thick white cord; it follows the turns 

 of the cochlea, but gives no considerable la- 

 teral fibrils to the walls of it. As to how the 

 auditory nerve ends in the membraneous laby- 

 rinth at this period nothing is known. 



In an anencephalous foetus described by 

 Hyrtl,t the cochlea was represented by a ca- 

 vity, from the base of which, corresponding to 

 the internal meatus, there rose a pyramid com- 

 posed of canals and extended to the roof of it. 

 This pyramid and the canals composing it were 

 the representative of the axis and its canals for 

 the transmission of the fibrils of the cochlear 

 nerve. There was no trace of a turn of the 

 cochlea nor of a lamina spiralis. 



We now enter into a more explored region, 

 viz. the progress of ossification in the laby- 

 rinthic shell, and for the knowledge we possess 

 on the subject we are chiefly indebted to the 

 late J. F. Meckel.J 



The osseous labyrinth is at first merely mem- 

 braneous; by-and-bye it becomes cartilaginous, 

 and lastly ossifies. The membraneous laby- 

 rinth has not been properly distinguished from 

 it. The former at first lies free in the cavity 

 of the cranium; the latter has never been ob- 

 served in an uncovered state. 



The development of the osseous labyrinth is 

 quite distinct from the formation of the bony 

 substance of the petrous bone. The latter com- 

 mences before the former. 



Ossification commences in the labyrinth to- 

 wards the end of the third month round the 

 fenestra rotunda, first at the upper part, then 

 at the lower part ; and when a ring of bone has 

 thus been produced, ossification extends for- 

 wards. At the same time that the process just 

 described takes place, another osseous nucleus, 

 quite distinct from the preceding, is developed 

 at the outer extremity of the superior vertical 

 semicircular canal ; there then appears a third 

 small scale nearly in the middle of the posterior 

 vertical semicircular canal. Proceeding from 

 the first nucleus of bone, ossification makes 

 rapid progress backwards and downwards; 



* Op. cit. p. 208. 

 t Op. et loc. cit. 

 $ Op. et loc. cit. 



whence the floor of the labyrinth is formed* 

 The second nucleus enlarges perhaps even 

 more quickly than the first, so that the whole 

 superior vertical semicircular canal is soon os- 

 sified, with the exception of its lower concave 

 surface. From the inner extremity of this se- 

 micircular canal ossification extends on the 

 inner surface of the petrous bone, circum- 

 scribes the internal auditory meatus, penetrates 

 into its interior, and forms the base of the 

 cochlea. In the fifth month ossification ex- 

 tends from the two first nuclei to the horizontal 

 semicircular canal. 



In the ossification of the cochlea, that of the 

 petrous bone has but a very small share. All 

 that the petrous bone contributes is merely a 

 thin prolongation which it sends between the 

 turns of the cochlea. This process is at first 

 broader than at a subsequent period. From 

 the third month, as the cochlea widens from 

 without inwards, the process in question be- 

 comes thinner, and, at the same time, are deve- 

 loped the less considerable projections which 

 separate externally the first external turn and a 

 half of the cochlea from each other. 



The ossification of the labyrinth has been 

 found imperfect; thus Krombholz* relates a 

 case in which he found the semicircular canals, 

 as well as both scalae of the cochlea, presenting 

 the same thinness of walls as is remarked in the 

 foetus. Some places were merely membraneous. 

 I have already mentioned that the aqueducts 

 are sometimes unusually wide, a circumstance 

 conceivable when we consider the mode in 

 which most likely they are developed, and 

 which was spoken of when considering them. 

 In certain of the lower animals, such as the 

 pig, they are naturally wide. 



At first the osseous labyrinth is quite dis- 

 tinct from the mass of the petrous bone, in 

 which it is, as it were, embedded. Its outer 

 surface is, up to the fifth month, quite smooth; 

 the corresponding inner surface of the osseous 

 mass of the petrous bone is smooth also, but 

 not so much so. The two surfaces are soon 

 confounded together,al though the spongy cellular 

 structure of the petrous bone can still, even for 

 some time after birth, be easily enough removed 

 from around the hard bony substance of the laby- 

 rinth. Afterwards they become inseparable, 

 though it is still possible to perceive a trace 

 of the line of demarcation between them, 

 especially in the cochlea. 



All the above circumstances show that the 

 osseous labyrinth, though in the petrous bone, 

 is not of it; and that, as has been already said, it 

 cannot be affirmed tobelongto the skeleton, but 

 to be merely embedded in a bone which does. 

 Moreover as Weberf remarks, the osseous laby- 

 rinth is not in all animals enclosed in the same 

 bone of the skull ; for in fishes, when a trace of the 

 osseous labyrinth is yet to be found, the semi- 

 circular canals, or the rudimentary representa- 

 tive of them, are situated in the occipital bone. 



* Miicke, kurze Uebersicht der gegenwaitig be- 

 stehenden Lehr-und-Erziehungsanstalten fur Taub* 

 stumme u. s. w. Prag. 1827, p. 19. 



t Hildebrandt's Anutomie. Bd. iv. p. 40. 



