A text-book on diseases of the ear, nose and throat . directed exactly outward, but is curved somewhat two foetal drum-cavities at this time appear like wing-shaped, lateralappendages of the pharynx. (Siebenmann.) At this time no parietal piece exists corresponding to the Eustachiantube. However, the increase in length of the tubo-tympanal space keeps equal pace with the rapid increaseFig. 23. in thickness in front of the laj^er of soft tissue surrounding thepharynx. In this way, as the audi-tory canal is inserted between theconcha and membrana tympani,the Eustachian tube inserts
A text-book on diseases of the ear, nose and throat . directed exactly outward, but is curved somewhat two foetal drum-cavities at this time appear like wing-shaped, lateralappendages of the pharynx. (Siebenmann.) At this time no parietal piece exists corresponding to the Eustachiantube. However, the increase in length of the tubo-tympanal space keeps equal pace with the rapid increaseFig. 23. in thickness in front of the laj^er of soft tissue surrounding thepharynx. In this way, as the audi-tory canal is inserted between theconcha and membrana tympani,the Eustachian tube inserts itselfafter the sixth week between thepharynx and middle ear. (Sie-benmann. ) The tympanum and the Eusta-chian tube, derivatives of the en-toderm, are to be regarded asdiflerentiations of the primitivepharyngeal cavity. The dorsalpart of the closing membrane ofthis cleft persists as the tympanicmembrane. The outer layer of the membrana originates from the ecto-derm, the inner layer from the entoderm, and the middle fibrous layerfrom the mesoderm. 24. Cast of the middle ear of new-born child, rightside. (Siebenmann.) 1, Eustachian tube; 2, attic ;3, antrum ; A, malleus ; 5, incus. ANATOMY AND PHYSIOLOGY OF THE TYMPANIC CAVITY. 25 Auditory Ossicles.—Tlie foundation of the stapes in the human embryoappears at the end of the fourth week as an irregular layer of blastemarich in cells. This lies in the dorsal wall of the first pharyngeal pouch,and externally is in continuous connection with the blastema correspond-ing at that point to the richly cellular, blending tract of the first andsecond branchial arches. The malleus and incus are not, even in the fifth week, indicated byany special grouping of cells. At the beginning of the sixth week, how-ever, the anterior cartilaginous structures of the first and second branchialarches differentiate, and simultaneously with the appearance of Meckelsand Eeicherts cartilaginous rods appear also indications of the malleusand the incus. Ossifi
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