The Ossification of the Temporal Bone
At birth the temporal bone consists of three parts easily separable in the macerated j skull : they are the petrosal, squamosal, and the tympanic. The styloid process is cartilaginous with the exception of its basal element, the tympano-hyal, which, with the ear-bones, will be described with the appendicular elements of the cranium. I The squamosal and tympanic bones develop in membrane. The squamosal is formed from one center, which appears as early as the eighth week. Ossification extends into the zygoma, which grows concurrently with the squamosal. At first the tympanic border is nearly straight, but soon assumes its characteristic horseshoe shape. At birth the post- glenoid tubercle is conspicuous, and at the hinder end of the squamosal there is a recess where it conies into relation with the mastoid antrum. The center appears for the tympanic bone about the twelfth week. At birth it is a horseshoe-shaped ossicle slightly ankylosed to the lower border of the squamosal; the open arms being directed upwards. The tip of the anterior arm terminates in a small irregular process, and the inner aspect presents, in the lower half of its circumference, 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 centers, or groups of centers, named according to their relation to the ear-capsule in its embryonic position.
The nuclei are deposited in the following order:
- 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 rotunda from above down- wards, and forms the floor of the vestibule, the lower part of the fenestra ovalis, and the internal auditory 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.
- 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.
- The pterotic nucleus ossifies the tegmen tympani and covers in the external semi- circular canal; the ossific matter is first deposited over the outer limb of this canal.
- The epiotic is the last to appear, and is first seen at the most posterior part of the posterior semicircular canal; often it is double. This center gives rise to the mastoid process.
At birth the bone is of loose and open texture, resembling biscuit or unglazed porcelain, 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 particulars. 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 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 tympanic annnlus grows rapidly and forms the tympanic plate. Growth in the tympanic bone takes place most rapidly from the tubercles at its upper 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 persists until puberty, and 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 auricular fissure. The anterior portion of the tympanic plate forms with the inferior border of the squamosal a cleft known as the Glaserian fissure, which is subsequently encroached upon by the growth of the petrosal. As the tympanic plate increases in size it joins the outer wall of the carotid canal and presents a prominent lower edge, known as the vaginal process.
The mastoid process becomes distinct about the first year, coincident with the obliteration of the petro- squamous suture. It increases in thickness by deposit from the periosteum. Towards puberty, rarely earlier, the process becomes pneumatic, the air-cells being lined by delicate mucous membrane. In old skulls the air-cells may extend into the jugular process of the occipital bone.
At birth the mastoid antrum is relatively large and bounded externally by a thin plate of bone belonging to the squamosal. As the mastoid increases in thickness the antrum comes to lie at a greater depth from the surface and becomes relatively smaller.
In 20 per cent, of skulls there are no air-cells in the mastoid process.