THE BONES OF THE HEAD 



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ance. The opisthotic centre appears on the promontory on the inner wall of 

 the tympanum, from which point ossification extends downwards around the 

 fenestra rotunda, and forms (i) the floor of the vestibule, (2) the lower part 

 of the fenestra ovahs, (3) the floor 01 the internal auditory meatus, (4) the 

 greater part of the bony investment of the cochlea, (5) the carotid canal, 

 and (6) the floor of the tympanum. The pro-Otic centre appears near the inner 

 hmb of the superior semicircular canal in the region of the eminentia arcuata, 

 and from it are formed (i) the bony investment of the superior semicircular 



Squaln.-Zyg. Portion 



Internal Auditory Meatus 



Fenestra Ovalis 



-Tympanic Annulus 



_,Floccular Fossa 



^ Aqueductus Vestibuli 



- '- Aqueductus Cochleae 



Squam.-Zyg. Portion 



Petrosal Portion 



(Petro-Mastoid) 



Pyramid, with opening for 

 _. _ . Stapedius 



~~ Sinus Tympani 



Fenestra Rotunda 



Tympanic Annulus 



Promontory 



Fig. 52. — The Temporal Bone in Early Life. 



A. Squamo-ZA'gomatic Portion and Tympanic Annulus; B, Petrosal Portion; 

 C, The Bone at Birth. 



canal, (2) the roof of the vestibule, (3) the roof of the cochlea, (4) the roof of 

 the internal auditory meatus, (5) the upper part of the fenestra ovalis, and 

 (6) the upper and inner part of the mastoid portion. The pterotic centre (Bland- 

 Sutton) appears over the outer limb of the external semicircular canal, and 

 from it are formed (i) the covering of the external semicircular canal, and 

 (2) the tegmen tympani. The eplotic centre, sometimes double, appears in 

 the region of the back part of the posterior semicircular canal, and from it the 

 lower part of the mastoid is formed, as well as the investment of the posterior 

 semicircular canal. At the period of birth (the tympanic having previously 

 joined the squamosal) the temporal bone is composed of two parts — (i ) a united 



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