At the level of the anterior acoustic foramen the cornu principalis of the hyale (corn. prin.) appears as a lateral ledge at the ventrolateral corner of the otic capsule ([Fig. 20a-b]). The cornu principalis diverges from the ledge at the level of the abbreviated bridge between the anterior and posterior acoustic foramina. In posterior sections the cornu lies medial to the squamosal-pterygoid process-pterygoid complex ([Fig. 18]). The posterior terminus of the cornu lies at a level with that of the posterior acoustic foramen.
The pars externa plectri (p. ext. pl., [Fig. 20a-b]) is cartilaginous and first appears dorsal to the ventral arm of the squamosal in association with the tympanic membrane. The pars externa plectri expands dorsomedially and is fused briefly to the crista parotica by the pars ascendens plectri (p. asc. pl., [Fig. 20b]). The pars interna plectri (p. int. pl., [Fig. 20b-f]) is cartilaginous and appears medial to the pars media plectri and the ventrolateral ledge of the otic capsule at the level of the anterior acoustic foramen. The pars media plectri (p. med. pl., [Fig. 20b-f]), a cartilage and bone element, appears proximally at the dorsolateral edge of the otic capsule and distally, ventral to the squamosal at a level between the anterior and posterior acoustic foramina. At the level of the posterior acoustic foramina the pars media plectri is bony, greatly expanded in size, and joined to the pars interna plectri medially.
The operculum (op., [Fig. 20d-h]) is cartilaginous and lies medial to the lateral edge of the otic capsule between the pars interna plectri and pars media plectri. The anterior end of the operculum ([Fig. 20d]) lies at a level corresponding to the posterior part of the posterior acoustic foramen. Posteriorly the operculum increases in size, and the pars interna plectri and pars media plectri are reduced ([Fig. 20e-f]). At a level corresponding to the posterior border of the posterior acoustic foramen the medial portion of the pars interna plectri disappears and leaves a small lateral rod of cartilage surrounded on all but the ventral side by the operculum ([Fig. 20f]). The operculum expands medially to merge with the main part of the otic capsule ([Fig. 20g]). The lateral edge of the operculum expands ventrally and then dorsomedially to form a complete tube. Slightly more posteriorly the cartilaginous lateral edge of the otic capsule, lateral to the operculum, dissipates into connective tissue and finally disappears, leaving the posterior end of the operculum as the most distal element of the otic capsule ([Fig. 20h]).
Fig. 20. Transverse sections through otic capsule: a) level of anterior ledge of otic capsule; b) anterior level of pars interna plectri and pars ascendens plectri; c) level of pars media plectri; d-f) successive levels of operculum and pars media plectri; g-h) posterior levels of operculum. Abbreviations: corn. prin., cornu principalis; cr. par., crista parotica; op., operculum; p. asc. pl., pars ascendens plectri; p. ext. pl., pars externa plectri; p. int. pl., pars interna plectri; p. med. pl., pars media plectri; sq., squamosal; tymp. r., tympanic ring; vl. l. ot. c., ventrolateral ledge of otic capsule.
Ossification in otic and occipital regions.—The otic region of the cranium is largely unossified. At the level of the optic foramen ([Fig. 14]) the floor of the neurocranium is cartilaginous but completely underlaid by the bony parasphenoid. The taenia tecti marginales and the tectum synoticum are covered dorsally and laterally by the frontoparietals. Perichondral ossification representing the prootic bone occurs at the margin of the optic foramen and somewhat posteriorly over part of the floor of the neurocranium. Perichondral and endochondral ossification occurs in the sides of the neurocranium ventral to the lamina perpendicularis. This ossification expands laterally until it meets the crista parotica dorsolaterally and forms the dorsal part of the prootic bone. The anteroventral edge of the otic capsule remains cartilaginous. Posteriorly, at the level of the anterior acoustic foramen, endochondral ossification is meager and restricted to the dorsomedial parts of the otic capsule, plus a small amount in the neurocranial floor; perichondral ossification is restricted to the peripheral areas showing endochondral ossification. Posteriorly, endochondral ossification is restricted in the dorsal part of the otic capsule but somewhat increased in the floor of the capsule. The lateral part of the otic capsule posterior to the terminus of the operculum and the ventromedial and dorsomedial parts of the neurocranium remain unossified.
Articular Region