150 



EAK 



all later stages, as a pocket named the anterior tympanic recess. From this, ii 

 a forward direction, a shallow groove extends along the roof of the pharynx, anc 

 another furrow runs backwards to the dorsal pocket of the second visceral pouch. 

 This furrow is divided into two portions, a short, sharply descending section and 

 shallow horizontal posterior prolongation. At the junction of these two a secom 

 pocket develops which is named the posterior tympanic recess. By the expansioi 

 of the pouch and the deepening of these furrows the primitive tympanum is lai( 

 down as a wing-like diverticulum from the pharynx, which extends in a horizonl 

 and then in a dorsal direction. In shape it is a narrow cleft, the inner wall bein^ 

 rendered salient by the growing cochlea. In the mesenchyme on its outer sid( 

 the cartilages of Meckel and of Reichert are laid down. The upper end of the forme 

 passes over the two recesses above named and expands to form the rudiments oi 

 the malleus and incus. From the malleus a process extends downwards an< 

 inwards which is the rudiment of its handle (fig. 190). This causes a projection intc 



vestibule 



M.c. 



FIG. 191. [RECONSTRUCTION OF THE AUDITOBY CAPSULE, THE TYMPANUM, AUDITORY OSSICLES. AND 

 EXTERNAL AUDITORY MEATUS OF A HUMAN EMBRYO 24'4 MM. LONG, FROM THE FRONT. (After 

 Hammar.) 



M.c . cartilage of Meckel (the proximal end of Meckel's cartilage shows two points, the malleus and 

 incus respectively) ; B.C., cartilage of Reichert; pr.a.m., the line points to the junction of the outer 

 portion developed as a pit from the surface and the inner portion developed from the meatal plate. 



the outer wall of the tympanum between the anterior and posterior tympanic 

 recesses. The primitive tympanum is cut off from the pharynx from behind 

 forwards until it opens only by a short tubal portion. At this stage the cut-off 

 tubo-tympanic cleft is directed nearly horizontally outwards. Its outer end, with the 

 two recesses, is obliquely placed, and overlaps the inner end of the meatal plate 

 above described, a layer of mesenchyme in which the handle of the malleus lies 

 intervening between them. This layer of mesenchyme gives rise to the mem- 

 brana propria of the drum, and the epithelium to its inner mucous covering. The 

 handle of the malleus lies between the two tympanic recesses. In later stages 

 the tubo-tympanic cleft comes to lie more and more antero-posteriorly as the 

 cranial base pushes forwards, and the short tubal portion becomes elongated, 

 until the adult position and relationships are attained. During the later months 

 of pregnancy the lining membrane of the tympanum becomes greatly thickened 

 and gelatinous, so that the epithelial lamellae are brought together and the lumen 



