THE TYMPANUM 



43 



irrcirular process, aud the inner aspect presents, in the lower half of its circumference, a groove 

 {or the reception of the tympanic membrane. 



Up to the middle of the fifth month the jicriotic capsule is cartihiirinous ; it then ossifies so 

 rapidly that by the end of the sixth month its chief portion is converted into porous bone. The 

 ossific material is deposited in four centres, or groups of centres, named according to their relation 

 to the ear-capsule in its embryonic position. 



The nuclei are deposited in the following order : — 



1. The opisthotic appears at the end of the filth month. The osseous material is seen first 

 on the promontory, and it quickly surrounds the fenestra rotunda from above downwards, and 

 forms the floor of the vestibule, the lower part of the fenestra ovalis, and the internal auditory 



Fig. 51. — The Temporal Bone at Birtit. (Outer view.) 



POST-GLENOID TUBERCLE 

 GLASERIAN FISSURE 



TYMPANIC FISSURE — 



SQUAMOSAL 



,»'.- ^ '' y ~ ',-'/" PETRO-SQUAMOUS SUTURE 



STYLO-MASTOID FORAMEN 

 TYMPANO-HYAL 



- CAROTID CANAL 



meatus ; it also invests the cochlea. Subsequently a plate of bone arises from it to surround the 

 internal carotid artery and form the floor of the tympanum. 



2. The pro-otic nucleus is deposited behind the internal auditory meatus near the inner limb 

 of the superior semicircular canal. It covers in a part of the cochlea, the vestibule, and the 

 internal auditory meatus,* completes the fenestra ovalis, and invests the superior semicircular 

 canal. 



3. The pterotic nucleus ossifies the tegmen tympani and covers in the external semicircular 

 canal ; the ossific matter is first deposited over the outer limb of this canal. 



4. The epiotic is the last to appear, and is first seen at the most posterior part of the posterior 

 semicircular canal ; it is often double. This centre gives rise to the mastoid process. 



At birth the bone is of loose and open texture, resembling biscuit or unglazed porcelain, thus 

 off"ering a striking contrast to the dense and ivory-like petrosal of the adult. It also diflfers from 



Fig. 



-Temporal Bone at Birth. (Inner view. 



HIATUS FALLOPII 

 FLOCCULAR FOSSA 

 AQUEDUCTUS VESTIBULI 

 INTERNAL AUDITORY MEATUS 



the adult bone in several other particulars. The floccular fossa is widely open and conspicuous. 

 A''<)ltolini has pointed out that a small canal leads from the floor of the floccular fossa and opens 

 posteriorly on the mastoid surface of the bone ; it may open in the mastoid antrum. The hiatus 

 Fallopii is unclosed, and the tympanic recess is filled with gelatinous connective tissue. The 

 mastoid process is not developed, and the jugular fossa is a shallow depression. 



After birth the parts grow rapidly. The tympanum becomes permeated with air ; the various 

 elements fuse ; and the tymjianic annulus grows rapidly and forms the tympanic plate. Growth 

 in the tymi)anic bone takes place most rapidly from the tubercles at its upjier extremities, and in 

 consequence of the slow growth of the lower segment a deep notch is formed ; gradually the 

 tubercles coalesce, leaving a foramen in the anterior part of the bony meatus which persi^ts until 

 puberty, and even in the adult. In most skulls a cleft capable of receiving the nail remains be- 

 tween the tympanic element and the ma.stoid process ; this is the auricular fissure. The anterior 

 portion of the tympanic plate forms with the inferior border of the .s(iuamosal a cleft known as 



