1562 A MANUAL OF ANATOMY 



to the epithelial wall of the primitive tympanic cavity. After the periotic 

 cartilaginous capsule has undergone ossification and formed the osseous wall 

 of the tympanum, the malleus, incus, stapes, and chorda tympani nerve lie 

 embedded in the mesodermic tissue which intervenes between the epithelial 

 or mucous roof of the membranous tympanum and its osseous roof. This, 

 mesodermic tissue disappears, and the mucous (orginally epithelial) lining of 

 the tympanum now comes into direct contact with the malleus, incus, stapes, 

 and chorda tympani nerve, all of which it encloses within folds. Though 

 these structures apparently lie within the tympanic cavity, this is not their 

 actual position. They are really outside the cavity, inasmuch as they lie 

 external to the mucous lining of the cavity. This may be illustrated by stating 

 that the handle or manubrium of the malleus and the chorda tympani nerve 

 do not lie in the tympanic cavity, but are placed between the middle, or 

 fibrous layer and the internal, or mucous, layer of the membrana tympani. 



External Ear. — The external ear consists of (i ) the external auditory meatus, 

 including the membrana tympani, and (2) the pinna. 



The external auditory meatus is developed from the upper part of the first 

 external or ectodermic furrow, which corresponds, in position, to the first 

 internal visceral cleft. 



The membrana tympani is developed from the closing membrane which 

 separates the first internal visceral cleft from the first external ectodermic 

 furrow. This closing membrane is a trilaminar structure. Its inner layer 

 consists of entoderm; its middle layer of mesoderm; and its outer layer of 

 ectoderm. The membrana tympani, which is developed from it, is therefore 

 trilaminar. The outer layer is ectodermic, and is continuous with the cuticular 

 lining of the external auditory meatus; the middle layer is mesodermic, or 

 fibrous ; and the inner layer is entodermic, or mucous, and is continuous with 

 the mucous lining of the tympanic cavity. 



The component parts of the pinna are developed from six projections, 

 consisting of mesoderm, covered by ectoderm. These appear on the upper 

 ends of the first and second visceral arches, where these bound the upper 

 part of the first external ectodermic furrow, which gives rise to the external 

 auditory meatus. The helix is developed from two of these tubercles, and 

 each of the other four gives rise to the antihelix, tragus, antitragus, and lobule^ 

 The mesodermic tissue of the projections becomes differentiated into con- 

 nective tissue and cartilage, and the ectoderm forms the integument. 



