The palatine (pal., [Figs. 1] [and 12]) lies in connective tissue medial and adjacent to the pars facialis. At its maximum size the palatine forms the bony posterior margin of the internal nares and extends dorsomedially from the pars palatina to the distal part of the solum nasi.
The nasal (nas., [Fig. 1]) is a thin bone overlying the tectum nasi anteriorly ([Fig. 7]). It expands laterally to form a complete roof over the cavum principale ([Fig. 10]). In the region of the internal nares, the nasal forms the lateral wall of the cavum principale ([Fig. 12]).
Fig. 13. Transverse section through sphenethmoid region at level of orbitonasal foramen. Abbreviations: ant. max. proc., anterior maxillary process; max., maxillary; orbnas. f., orbitonasal foramen; pasph., parasphenoid; spheth., sphenethmoid.
Sphenethmoid Region
Posterior to the transition zone between the planum antorbitale and solum nasi, the sphenethmoid (spheth., [Fig. 1]) is fully ossified medially, the lateral parts of the bone at this level are only ossified perichondrally. The septum nasi persists at the anterior level of the orbit and terminates just anterior to the orbitonasal foramen (orbnas. f.) and the anterior end of the parasphenoid (pasph., [Fig. 13]). The orbitonasal foramen is moderately large, has a complete bony margin, and is located at the dorsolateral corner of the braincase.
At the level of the orbitonasal foramen, the sphenethmoid is entirely ossified except for a small dorsolateral extension. This distal extension expands laterally in posterior sections as the braincase is increased to its maximum width at the mid-length of the orbit; the cartilaginous margin is retained throughout the length of the sphenethmoid.
The bony dorsomedial part of the sphenethmoid diverges, forming the anterior border of the frontoparietal fontanelle (fpar. fon., [Figs. 1] [and 14]). The entire fontanelle is covered with a layer of dense connective tissue continuous with that in which marginal bones and cartilage of the sphenethmoid lie, and which is discrete from the lower dermal layer of the overlying skin. At this level the braincase is U-shaped in cross-section. Ossification terminates first in the ventrolateral corners, followed by the lateral and dorsolateral areas. The bony support of the latter area is furnished by the lamina perpendicularis (lam. perp.) of the frontoparietal (fpar., [Fig. 14]). Cartilage appears in the ventral part of the sphenethmoid in posteromost sections; at the posterior levels of the orbit the sphenethmoid is entirely cartilaginous.