ORGAN OF HEARING. 



559 



B. Development and irregular conditions of 

 the tympanum and external ear. 



1. Of the tympanum and its contents. 



1. The cavity of the tympanum. A prolon- 

 gation or diverticulum of the mucous mem- 

 brane of the throat, extending to the periphe- 

 ral surface of the labyrinth, and forming, with 

 its blind end, a dilatation there, gives us the 

 simplest idea of a tympanum and Eustachian 

 tube. According to Huschke* and his views 

 are more or less supported by Burdach,f 

 Rathke,t and Valentin, the cavity of the tym- 

 panum with the Eustachian tube is a metamor- 

 phosis or remains of the first branchial fissure. 

 Hence, in its origin, the tympanum has nothing 

 in common with the labyrinth. 



Valentin|| found the Eustachian tube and 

 cavity of the tympanum, in an embryo at the 

 seventh week, under the form of a conical or 

 pyramidal fossa. This fossa gradually extends 

 into a tube, at first short and wide, but after- 

 wards longer and narrower in proportion as the 

 cavity of the tympanum becomes developed, 

 and recedes from the cavity of the mouth. 

 The Eustachian tube is at first simply membra- 

 neous. About the middle of pregnancy, ac- 

 cording to Meckellf and Burdach,** the third 

 month according to Valentin, it acquires a 

 cartilaginous investment. 



The blind membraneous pouch which repre- 

 sents the cavity of the tympanum is at first 

 very small and contracted, and is to be distin- 

 guished from the walls of the tympanum, which 

 it invests. The peripheral surface of the laby- 

 rinth forms, as is known, the inner solid wall 

 of the tympanum, and the tympanic ring and 

 membrana tympani the outer wall. In pro- 

 portion, therefore, as these parts, and also the 

 mastoid process are developed, so does the tym- 

 panic cavity acquire its proper form, and is 

 more and more withdrawn, both from the cavity 

 of the mouth and the lateral surface of the 

 head, with the integument of which it is, at an 

 early period, in contact. 



The prolongation of the mucous membrane 

 of the pharynx forming the Eustachian tube 

 and cavity of the tympanum, is at first very 

 vascular, soft, and loose, like the mucous mem- 

 brane of the nasal and guttural cavities ; but 

 after birth it loses its vascularity, and assumes 

 a more simple character. United by loose 

 cellular tissue to the subjacent osseous" wall, it 

 applies itself over all the elevations, and dips 

 into the depressions of it, and moreover forms 

 folds in which the ossicles are enveloped. The 

 cavity of the tympanum is in the foetus filled 

 with a mucus, sometimes transparent, some- 



* Isis von Oken, Jahrg. 1827, p. 401 ; Jahrg. 

 1828 p. 161; Jahrg. 1831, p. 951 ; and Meckel's 

 Archiv fur Anatomic und Physiologic, 1832, p. 



t Die Physiologic als Erfahrungswissenschaft. 

 Bd. ii. p. 465. 



J Anat. physiol. Untersuchungen uberden Kiem- 

 enapparat nnd das Zungenbein, 1832, p. 120. 



Handbuch der Entwickelungsgeschichte des 

 Menschen, p. 211. 



j| L. c. p. 211 and 212. 



^ Manuel d'Anatomie, &c. torn. iii. s. 1948, p. 



"Op. cit. Bd.ii. p.465. 



times bloody, and varying from the consistence 

 of water to that of a thick jelly. 



The cavity of the tympanum has been found 

 sometimes unusually contracted ; sometimes, 

 on the contrary, very much dilated. Non-de- 

 velopment of the Eustachian tube would be 

 necessarily attended by non-development of the 

 tympanum, but, not contrariwise. Absence 

 of the Eustachian tube and cavity of the tym- 

 panum has only been found in foetuses, in other 

 respects monstrous. 



In describing the development of the osse- 

 ous labyrinth, it has been already mentioned 

 that its peripheral surface, which forms the 

 inner solid wall of the tympanum, begins to 

 be ossified towards the end of the third month. 

 An ossific point first appears on the promontory 

 at the circumference of the cochlear fenestra, and. 

 gradually extends upwards, downwards, for- 

 wards and backwards. At this time there 

 is no trace of mastoid process. In the fourth 

 month the sinuositas mastoidea begins to ap- 

 pear, and the cavity of the tympanum be- 

 comes somewhat wider. The aqueduct of Fal- 

 lopius is not yet ossified, nor the canals for the 

 muscles of the stapes and malleus, a state of 

 parts found permanent in most of the lower mam- 

 mifera, and also frequently met with in the irre- 

 gular conditions of the ear in man. When the 

 cochlea is arrested in its developement, the pro- 

 montory is small in proportion or entirely 

 wanting. 



In the fifth month the aqueduct of Fallopius 

 and the canals for the muscles of the stapes and 

 malleus are ossified. In this month, also, the 

 vestibular fenestra is found completely formed, 

 and appears proportionally larger than in the 

 adult. In the sixth month, the temporal bone 

 being altogether more developed, and the mas- 

 toid process having begun to appear, the cavity 

 of the tympanum increases in capacity, espe- 

 cially its upper part, and the sinuositas mas- 

 toidea. The direction and situation of the 

 cochlear fenestra vary much at the different 

 periods of formation, which is owing chiefly to 

 the degree of development of the promontory. 



The vestibular and cochlear fenestrae are 

 sometimes found unusually small, or even en- 

 tirely wanting, the latter being obliterated by 

 an extension of osseous substance, the former 

 by the same cause, or by anchylosis of the 

 base of the stapes. The cochlear fenestra some- 

 times appears to open into the vestibule, but this 

 is when the cochlea is in a very rudimentary 

 state. Such a condition may be compared with 

 the state of parts found in the bird's ear. 



Little or nothing is known of the origin and 

 development of the membrana tympani. It 

 may be looked upon as the persistence of that 

 septum which exists at an early period at 

 the opening of all mucous canals, and which 

 is produced by the meeting of the indentation 

 inwards of the skin with the diverticulum of 

 the mucous cavity of the blastoderma. The 

 membrana tympani is larger in proportion, and 

 more vascular the younger the foetus is. The 

 form, situation, and direction of it in the foetus 

 is dependent on the tympanic ringr, and is quite 

 different from what is found in the adult. In 

 regard to form, it is rounder. As to situation 



