MIDDLE EAR 



149 



The primary ganglion is closely applied to the auditory vesicle, and the peripheral 

 nerves are very short. The secondary ganglia become included in the capsule as 

 this develops round the labyrinth. 



ACCESSORY PARTS OF THE ORGAN OF HEARING : MIDDLE EAR 

 AND EXTERNAL AUDITORY MEATTJS. 



The middle ear and the Eustachian tube are derived from the first 

 branchial pouch of the pharynx ; the auricular fossa and the external 



meatus from the first branchial cleft ; the former are therefore entodermic, 

 the latter ectodermic derivatives. l 



The bottom of the ectodermic cleft, which is shallow above and deeper below, 

 comes into contact for a time with the entoderm of the corresponding pharyngeal 

 pouch and its dorsal prolongation. The original depression persists as the fossa 

 of the auricle (concha and upper auricular fossa), while the different folds of that 

 structure are produced by a series of elevations which appear on the prominent 



concha 



incus malleus 



cochlea 



pharynx 



FIG. 190. RECONSTRUCTION. OF THE TYMPANUM, PRIMITIVE EXTERNAL AUDITORY MEATUS, COCHLEA, 



AND OSSICLES OF A HUMAN EMBRYO 24 MM. LONG, FROM THE FRONT. (After Hammar.) 



The tympanic cleft (tymp.} is seen extending from the pharynx; at its outer end is a notch 

 bounded by two recesses of the tympanic cavity, of which only the anterior is seen ; opposite the 

 notch is the handle of the malleus. Meckel's cartilage (M.c.} and Reichert's cartilage (M.c.) are cut 

 across at their lower ends : the former is directly continuous with the rudiments of the malleus and incus. 

 /ir.n.m., primitive external meatus. The spoon-shaped inward part of this is the meatal plate. 



lips of the fissure (see Section I. p. 88). The external auditory meatus is in part 

 produced from an inward tubular prolongation of the lower and deeper part of 

 the cleft, and in part from a solid epithelial plate which grows obliquely from it 

 inwards and downwards, below the fissure representing the tympanic cavity 

 (fig. 190). The cartilaginous portion of the meatus and also a small part of the 

 roof of the osseous meatus, which have a typical skin-lining, are derived from 

 the tubular invagination ; the deep portion of the osseous meatus is produced by 

 the shedding of the central cells of the epithelial plate. The lumen thus pro- 

 duced lies obliquely like the solid plate, and its upper and inner wall forms ultimately 

 the ectodermic covering of the tympanic membrane. 



The primitive tympanic cavity is derived from the dorsal prolongation of the 

 first visceral pouch. This and its lateral expansion are at first in contact with 

 the ectoderm, but the epithelial layers are soon separated again by mesenchyme. 

 The extremity of the dorsal prolongation is to be recognised at this stage, and through 



1 The following account is based on the very^detailed descriptions given by J. August Hammar 

 ( i'psala), Archiv. f. mikr. Anat. lix. 1902. 



