ORGAN OF HEARING. 



557 



III. 



1. Development and irregular conditions of the 



organ of hearing. 



A. Development and irregular conditions of 

 the ear-bulb. Our knowledge of the early for- 

 mation of the ear-bulb is not very precise. This 

 much we know, that it has quite a separate 

 origin from the rest of the apparatus of hearing. 

 Hence, in the irregular conditions of the organ 

 depending on defective development, there is 

 no constant and necessary relation betwixt the 

 labyrinth and the accessory parts of the ear; 

 for the latter may be imperfect while the former 

 is in its natural state, and vice versa. In many 

 cases, however, it has been found that imper- 

 fect development of the one attended an irre- 

 gular condition of the other. The earlier a 

 part is formed the fewer deviations it is subject 

 to, so a greater number of malformations affect 

 the accessory parts than the ear-bulb, as the 

 former are developed subsequent to the latter. 



The development of the ear-bulb commences 

 very early, soon after the appearance of the 

 eye. It takes place by the springing forth of 

 the auditory nerve in the form of a tubular 

 prolongation of the brain. At its central ex- 

 tremity the cavity of the cerebral prolongation 

 is continuous with that of the fourth ventricle. 

 Its peripheral extremity, which extends into 

 the muscular layer of the embryo, and particu- 

 larly into the osseous part of it, forms a vesi- 

 cular dilatation which is gradually separated 

 from the brain. To this vesicle of nervous 

 substance, which is the labyrinth, there grows 

 inwards a reflection of the tegumentary layer to 

 form the accessory parts of the organ of hearing. 



Such is Baer's account of the development 

 of the ear in the chick. Huschke, on the con- 

 trary, says that the membraneous labyrinth does 

 not arise from the brain, but is originally a 

 blind sac of the skin with an excretory duct, 

 which gradually contracts, until the blind sac 

 of skin is completely cut off from the rest of 

 the tegumentary system. 



However this may be, the labyrinth, accord- 

 ing to the observations of Valentin* made on 

 the embryo of the sheep, exists at a very early 

 period, under the form of a simple elongated 

 tube with an oblong cavity, which is the ves- 

 tibule. This cavity becomes broader, assumes 

 a rounder form, and presents in the interior a 

 somewhat uneven surface. Soon after this, its 

 inner end is elongated and begins to make a 

 circular turn, which is the first rudiment of the 

 cochlea. The turns of this cochlear vesicle be- 

 come gradually more developed. A short time 

 after the commencement of the development of 

 the cochlea, the semicircular canals begin to 

 show themselves as processes or diverticula of 

 the vestibule. There first appears the posterior 

 over and behind the vestibular fenestra ; it be- 

 comes elongated from within and below out- 

 wards and upwards, then bending in the form 

 of an arch returns to the vestibule. In a si- 

 milar manner the superior and inferior semi- 

 circular canals are formed. The semicircular 



* Handbuch der Entwickelungsgeschichte des 

 Menscheu, p. 206. 



canals are at first proportionally rery wide, but 

 gradually contract, till at last the ampullae pre- 

 sent the only trace of their former width. The 

 vestibule itself has diminished in breadth and 

 length, and acquired a more trapezoidal form. 

 The vestibular fenestra, which was before not 

 very distinct and still round, has become more 

 evident and exhibits an oval shape. 



What Valentin here describes is only the 

 basis of the future osseous labyrinth. There 

 exist as yet no observations bearing on the mode 

 of development of the membraneous labyrinth. 



The irregular conditions which the labyrinth 

 has been found to present, as well as the struc- 

 ture permanent in the lower animals, corre- 

 spond in a remarkable manner with the above-de- 

 scribed stages of development. Thus in a mon- 

 strous foetus, Hyrtl* found, instead of a vestibule, 

 cochlea, semicircular canals, and internal mea- 

 tus, a single very capacious cavity, containing a 

 membraneous sac, in which the auditory nerve, 

 sufficiently well developed, terminated. There 

 was no trace of vestibular or cochlear fenestra, 

 and the accessory parts of the organ were en- 

 tirely wanting. Rodererf describes a somewhat 

 similar case, in which, however, some of the 

 accessory parts presented themselves, though 

 in a very rudimentary and imperfect form. 



The cochlea has presented itself as a mere 

 subdivision of the vestibule without any wind- 

 ings, a state of parts which is permanent in 

 birds. In other cases, though presenting wind- 

 ings, these have been found fewer than natural, 

 and sometimes the spiral lamina has been 

 wanting or not extending throughout all the 

 turns of the cochlea, so that no subdivision 

 into scalae or but a very imperfect one pre- 

 sented itself. The semicircular canals are 

 sometimes smaller and narrower than usual; 

 one or all of them have been found wanting or 

 but partially present. In the latter case, after 

 running a short way, they have been observed 

 stopping short and terminating in a cul-de-sac. 

 The semicircular canals, as they are formed 

 later, more frequently present deviations from 

 the regular structure than the vestibule and 

 cochlea. 



Our knowledge of the ear-bulb in the human 

 embryo commences at about the third month, 

 when the membraneous labyrinth is already 

 very perfectly developed and surrounded by a 

 cartilaginous shell, having a structure as com- 

 plicated as at a more advanced period the bony 

 shell presents. The membraneous labyrinth is 

 at this early period so firm that it is not very 

 difficult, by means of careful dissection and 

 manipulation, to extract the whole from its 

 cartilaginous case. 



According to Meckel,J the membraneous la- 

 byrinth is composed at first of two perfectly 



* Beitrage zur pathologischen Anatomie des 

 Gehororgans. In the Medicinische Jahrbiicher, 

 des k. k. oestr. Staates. Wien, 1836. Bd. xx. 

 p. 446. 



t Descriptio foetus parasitic!. In Commentariis 

 Soc. reg. Goettingensis. torn. iv. 1754, pp. 136 



\ Manuel d'Anatomie, etc. traduit par Jourdan 

 et Breschet, tome iii. s. 1948 3 Paris, 1825. 



