1526 



HUMAN ANATOMY. 



The air-cells of the temporal bone, including those of the mastoid process, are formed 

 later by a process of absorption. 



The tympanic membrane results principally from changes which take place in the first 

 branchial arch ; it is originally thick and consists of a mesoblastic middle stratum, covered on its 

 outer surface by the ectoblast and on its inner surface by the entoblast 



Development of the External Ear. The median portion of the ectoblastic groove of the 

 first branchial furrow becomes deepened to form the outer part of the external auditory canal, 



FIG. 1280. 



- ?/- -cochlea 



10 vilEEKS +. 



Diagram illustrating development of human membranous cochlea; primary otic vesicle subdivides into vestibular 

 and cochlear pouches and endolymphatic appendage ; cochlear pouch becomes ductus cochlearis ; from vestibular 

 pouch are derived utricle, saccule and semicircular canals ; whilst endolymphatic appendage gives rise to endo- 

 lymphatic sac and duct. (Slreelfr.) 



while the surrounding parts of the first and second arches develop into the auricle. About the 

 fourth week of fcetal life, the thickened posterior margin of the first arch is broken up into three 

 tubercles by two transverse furrows. Similarly on the adjoining margin of the second arch, a 

 second vertical row of three tubercles is formed and, in addition, behind these a longitudinal 

 groove appears marking off a posterior ridge. From these six tubercles and the ridge are differ- 

 entiated the various parts of the auricle, the lowest nodule of the first arch becoming the tragus, 

 t!u remaining ones with the ridge giving rise to the helix, whilst from the three tubercles of 

 the second arch are developed, from above downward, the antihelix, the antitragus and the 

 lobule. 



