AUDITORY OSSICLES 



259 



masses, from the first continuous over the first branchial cleft, become separated from 

 the visceral skeleton, and form the cartilage of the outer ear. The proximal 

 portion of the mesial thickening in the mandibular arch does not develop farther, 

 but the distal portion forms the chondroblast of MeckeVs cartilage. Its upper 

 end early enlarges and becomes the rudiment of the malleus. The proximal part 

 of the mesial thickening in the hyoid arch becomes thickened round the stapedial 

 artery, and gives rise to the primitive stapes. This is connected with the incus by 

 a strand of the common blastema which persists as the rudiment of the long process 

 of that bone. The distal part of the mesial portion of the hyoid blastema becomes 

 the hyoid bar. It is at first connected with the stapes by a membranous strand 

 (pars interhyalis], which soon disappears, so that the stapes is separated from the 

 rest of the hyoid arch. 



In these chondroblast areas cartilage is now developed. Meckel's cartilage 

 is deposited as a single piece. Its enlarged upper end is the malleus, which has 

 meantime developed a projection which becomes its handle, and another which 

 becomes its external process. Between the malleus and incus, which chondrifies 



head of malleus 



facial nerve 



stapes 



cartilage of Meckel 



pro. br. of malleus 



tympanic ring 

 handle of malleus 

 chorda tympani 



cartilage of ReicJtert 

 'acial nerve 



FIG. 313. RECONSTRUCTION OP THE SAME PARTS AS ARE SHOWN IN FIGS. 311, 812 OP AN 



EMBRYO OF 55 MM. LONG ; LEFT SIDE, INNER ASPECT. (After Bromail.) 



from a separate centre, there is at first a membranous layer which is afterwards 

 absorbed to give rise to the joint between the bones. In the same manner the 

 original union between the stapes and the long process of the incus is broken to 

 form the joint between them. The incus in its chondroblastic stage becomes 

 attached to the auditory capsule, but is separated from it again when the cartilage 

 is formed, the original intervening tissue becoming the ligament of the bone. 

 The stapes is at first free from the ear-capsule, but later becomes attached to it 

 and ultimately, when in the end of the second month it begins to lose its ring-shape 

 and assume the adult form, the base takes form in the fenestra ovalis, where the wall 

 of the capsule is reduced to a layer of perichondrium over it. The stapes and hyoid 

 cartilage chondrify separately, as does also the laterohyal cartilage mentioned 

 above. This becomes fixed to the ear-capsule, and by an extension of the cartilage 

 between it and the hyoid cartilage the latter obtains a direct secondary attachment 

 to the capsule. The bar thus formed becomes the styloid process. Its basal part 

 lies in the wall of the tympanum and takes part in the formation of the facial canal. 

 Its lower part is represented by the styloid ligament and lesser cornu of the hyoid 

 bone. The ossicles ossify each by a single centre. When the malleus has become 



s2 



