560 



ORGAN OF HEARING. 



the osseous portion of the auditory passage 

 not being yet developed, the membrana tympani 

 is found at first closer to the surface than after- 

 wards; and its direction is oblique from above 

 downwards, and from without inwards, so that 

 it has a more or less horizontal position, corres- 

 ponding to the base of the cranium. 



More is known of the origin and develop- 

 ment of the tympanic ring in which the mem- 

 brana tympani is framed. It appears later 

 than the membrana tympani and the ossicles. 

 A specimen of the tympanic ring before me, 

 which was removed from an embryo at about 

 the third month, is an incomplete ring of bone 

 about one-tenth of an inch in diameter. It is 

 about the thickness of a hair, except at its 

 anterior extremity, which is broad and flat like 

 a spatula, for the extent of about one-twelfth 

 of an inch. The groove for the membrana 

 tympani can be perceived with a magnifying 

 glass. From the fifth month the tympanic ring 

 is found more or less adherent to the rest of 

 the temporal bone. In the lower Mammifera 

 there are three parts developed from the tym- 

 panic ring, viz. 1, the groove for the mem- 

 brana tympani, 2, the bulla ossea, and 3, the 

 osseous part of the auditory passage. In 

 birds the tympanic ring is represented by the 

 os quudratum. In man there is no bulla ossea, 

 only the groove for the membrana tympani and 

 the osseous part of the auditory passage. The 

 side of the tympanic ring external to the groove 

 shoots out to form the osseous part of the 

 external auditory passage, but so slowly that 

 from the second to the sixth or seventh year, 

 the lower surface of the auditory passage is 

 still cartilaginous, although the outer orifice to 

 which the cartilaginous part of the auditory 

 passage is fixed is already ossified. About the 

 twelfth year the passage is closed in by bone, 

 and becomes quite complete towards manhood. 

 The inner surface of the ring grows a little 

 at the lower part, and helps, together with a 

 process which extends from the petrous bone, 

 to form the lower wall of the tympanic cavity. 

 It is to be remarked that this inner part of the 

 tympanic ring always remains distinct and is 

 never changed so much as the external. 



Fig. 261. 



The tympanic ring, and the membrana tympani framed 

 into it. The handle of the matteus is seen shining 

 through ttue membrane. 



The whole outer wall of the cavity of the 

 tympanum has been found wanting in a mon- 

 strous foetus. Hyrtl, who mentions the case, 

 says that the cavity of the tympanum itself 

 was represented only by a very shallow depres- 



sion in the petrous bone, in which the skin of 

 the auditory passage formed a cul-de-sac. The 

 Eustachian tube was present. Hyrtl mentions 

 another case in which the tympanic ring was 

 much smaller than usual, and in which the 

 membrana tympani presented in the direction 

 of one of its radii, a large opening as if a 

 piece had been cut out. The so-called hiatus 

 Riviniunm ought, perhaps, to be looked upon, 

 as Huschke observes, as a defect in original 

 formation. The membrana tympani has been 

 sometimes found congenitally too large, some- 

 times too small, sometimes of an elongated 

 form, sometimes of a triangular form. A 

 thickening and parchment appearance of the 

 membrane, or ossification of it to a greater or 

 less extent, if not always, appears to be more 

 usually an acquired malformation. 



2. The small bones of the tympanum. The 

 small bones are formed at a very early period. 

 The malleus and incus appear before the 

 stapes. The two former, according to Rathke 

 and Valentin, appear like a small wart growing 

 out from the posterior wall of the tympanum. 

 The stapes is like a growth from the outer sur- 

 face of the labyrinth ; it appears as a small 

 pyramidal wart flattened on the sides and thin, 

 in the middle, lying, according to Rathke, in 

 a small funnel-like depression, the bottom of 

 which is the future vestibular fenestra. 



According to Weber* the ossicles are not 

 developed in the cavity of the mucous mem- 

 brane of the tympanic cavity, but in a sac 

 which is a continuation of the dura mater, and 

 comes through a fissure between the petrous 

 bone and the squamous portion of the tem- 

 poral into the tympanic cavity. This situation 

 of the ossicles at an early period corresponds 

 with that of those discovered by Weber in the 

 fishes already mentioned. By this mode of 

 development, as far as regards situation, 

 may be explained the dislocated state of the 

 ossicles so frequently found in the irregular 

 conditions of the ear. 



According to Meckel,f the ossicles are at 

 the commencement of the third month pro- 

 portionally very large, though still cartilaginous, 

 and the stapes not to be distinguished from the 

 incus. Thus, for example, the length of the 

 malleus in a foetus of the fourth month amounts 

 to three lines, whilst the length of the body 

 from the vertex to the coccyx measures four 

 inches, hence the lenght of the malleus is to 

 that of the trunk as one to sixteen ; whereas 

 in the adult the proportion is only as one to 

 ninety, the malleus being four lines long, and 

 the distance between the vertex and the coccyx 

 amounting to two-and-a-half feet. At birth 

 the ossicles are as large as in the adult. 



Ossification of the small bones commences, 

 according to Burdaeh,J about the twelfth week. 

 Rathke and Valentin agree with Meckel, that 

 ossification begins first and at the same time in 

 the malleus and incus, and only afterwards in 

 the stapes. In the malleus the first point of 

 bone appears on the head, a second at the root 



* Hildebrandt's Anatomic, Band. iv. p. 39. 

 t Op. cit. torn. iii. p. 197. s. 1948. 

 $ Op. cit. Band. ii. p. 384. 



