218 Anatomy of Skeleton 



small bony eminence containing the Stapedius muscle, whose tendon emerges from its apex : to 

 the outside of this and a little lower the Her chorda posterius allows the chorda tympani to leave 

 the facial canal. The tympanic branch of the ninth nerve comes through the floor just below the 

 promontory, on to which the nerve runs and ramifies. 



The promontory marking the position of the cochlea, it follows that the fenestrae must be in 

 the region of the bony vestibule, into which they open, and the semi-circular canals must lie behind 

 this. These canals cause prominences on the surfaces of the bone, the ampulla of the external 

 canal showing in the aditus, above and behind the facial canal, the upper curve of the superior 

 semi-circular canal making the arcuate eminence, and the upper arc of the posterior one causing a 

 rounded elevation between the subarcuate fossa and the aquceductus vestibuli on the inner aspect of 

 the bone. 



An incomplete bony septum separates the bony tube from the canal of the Tensor tympani, 

 ending behind in a hook-like process round which the tendon of the muscle turns. 



Development. 



At birth the bone (Fig. 214) is practically in the condition shown schematically 

 in the fifth and sixth drawings in Fig. 172. The four elements of the bone are easily 

 separated, the styloid process teing as a rule cartilaginous, although its embedded 

 part is tony. The tympanic plate is represented by the incomplete ring,. so that there 

 is no bony meatus, but the membrane is practically on the surface of the bone. It is 

 unnecessary to give a further account of the origin of the four parts of the bone already 

 illustrated in Fig. 172, and it only remains to deal with the ossification : this occurs 

 in membrane in the squama and tympanic ring, but in cartilage in the petrous and 

 styloid. 



Squama. 



One centre, above the root of the zygoma, about the seventh or eighth week : some 

 observers maintain that there are two additional centres, one above and one behind 

 this. 



Tympanic Ring. 



One centre, appearing in the middle of the third month in the thick tissue by 

 the wall of the cavity and extending from this backward and upward to make the ring. 



The Petrous. 



This ossifies from four or more centres that appear in the fifth month and are 

 indistinguishable as separate centres in the sixth month. These are : 



(1) Opistholic : the first to appear, forms that part of the petrous that lies below 

 the level of the internal meatus : this includes the lower part of the inner wall of the 

 tympanum and, in front of this, the carotid canal. 



(2) Pro-otic, the main centre, forming the part of the petrous that lies above the 

 internal meatus, as far forward as the apex and back to the mastoid region : the cerftre 

 appears near the eminentia arcuata. 



(3) Pt erotic : a small centre responsib'e for the roof of the tympanum and tegmen 

 tympani ; there may, however, be a separate centre for the latter. 



(4) Epiotic : one or more from which the mass of the mastoid region is ossified. 



Styloid. 



The tympano-hyal centre makes its appearance late in foetal life : the stylo- 

 hyal centre appears shortly after birth. The process remains attached by cartilage 

 until nearly middle life, when it unites with the tympano-hyal. 



