76 



THE SKELETON 



in a small irregular process, and the medial aspect presents, in the lower half of its circumfer- 

 ence, a groove for the reception of the tympanic membrane. 



Up to the middle of the fifth month the periotic capsule is cartilaginous; 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 fifth month. The osseous material is seen first 

 on the promontory, and it quickly surrounds the fenestra cochleae from above downward, and 

 forms the floor of the vestibule, the lower part of the fenestra vestibuli, and the internal au- 

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

 round the internal carotid artery and form the floor of the tympanum. 



2. The prootic nucleus is deposited behind the internal auditory meatus near the medial 

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

 the internal auditory meatus, completes the fenestra vestibuli, and invests the superior semi- 

 circular canal. 



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

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



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

 part of the posterior semicircular canal. 



At birth the bone is of loose and open texture, thus offering a striking contrast to the dense 

 and ivory-like petrosal of the adult. It also differs from the adult bone in several other par- 

 ticulars. The floccular fossa is widely open and conspicuous. Voltolini 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 canalis facialis is unclosed 



Fig. 96. — Temporal Bone at the Sixth Year. 



External auditory meatus 



Non-ossified area of the tympanic plate 



Petro-tympanic fissure 



. i_ Wormian bone in the 



1 ^ parietal notch 



and the tympanum 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 var- 

 ious elements fuse, and the tympanic annulus grows rapidly and forms the tympanic plate. 

 Development of the tympanic plate takes place by an outgrowth of bone from the lateral 

 aspect of the tympanic annulus. This outgrowth takes place most rapidly from the tubercles 

 or spines at its upper extremities, and in consequence of the slow growth of the lower segment a 

 deep notch is formed; gradually the tubercles coalesce, lateral to the notch, so as to enclose a 

 foramen which persists until puberty, and sometimes even in the adult. In most skulls a cleft 

 capable of receiving the nail remains between the tympanic element and the mastoid process; 

 this is the tympano-mastoid fissure. The anterior portion of the tympanic plate forms with 

 the inferior border of the squamosal a cleft known as the petro-tympanic fissure, which is sub- 

 sequently encroached upon by the growth of the petrosal. As the tympanic plate increases in 

 size it joins the lateral wall of the carotid canal and presents a prominent lower edge, known as 

 the vaginal process (shcatli of the styloid). 



The mastoid process Ijccomes distinct about the first year, coincident with the obliteration 

 of the petro-sfiuamoiiH suture, and increases in thickness by deposit from the periosteum. 

 According to most writers, the process becomes pneumatic about the time of puberty, but 

 it has been shown by "i'oung and Milligan that the mastoitl air-cells develop at a much 

 earlier period than is usually supposed. These writ(MS have described specimens in which the 

 air-cells were present, as small pit-like diverticula from the mastoid antrum, in a nine months' 

 fcEtus and in an infant one year old. In old skulls the air-cells may extend into the jugular 

 process of tlio occif)ital bone. 



At birth the mastoid antrum is relatively large and bounded laterally by a thin plate of 

 bone belonging to the Hfjuamosal (post-auditory i)rocess). As the mastoid increases in thick- 

 ness the antrum comes to lie at a greater depth from the surface and becomes relatively smaller. 



