Figs. 6-7. Transverse sections through olfactory capsule: 6) posterior level of cavum medium; 7) anterior level of prevomer. Abbreviations: cav. inf., cavum inferius; cav. med., cavum medium; cav. prin., cavum principale; cr. sub., crista subnasalis; ext. nar., external nares; inf., infundibulum; inf. pnas. c., inferior prenasal cartilage; l. inf., lamina inferior; l. sup., lamina superior; max., maxillary; nas., nasal; ncl. dt., nasolacrimal duct; obl. c., oblique cartilage; p. fac., pars facialis; p. pal., pars palatina; pvom., prevomer; rec. med., recessus medialis; sept. nas., septum nasi; sol. nas., solum nasi; spmax., septomaxillary; tect. nas., tectum nasi.
Nasal cavities and associated structures.—The cavum principale (cav. prin., [Fig. 4]) is the most anterior of the nasal cavities. It first appears within the capsule bordered dorsally by the tectum nasi, medially and ventrally by the septum nasi, and laterally by the alary cartilage. The cavity extends posteriorly within recesses of the sphenethmoid to the level at which the septum nasi terminates.
The cavum medium (cav. med., [Fig. 5]) lies ventral and slightly posterior to the anterior end of the cavum principale. It appears slightly anterior to the septomaxillary at the level of the foramen ramus externus narium and ramus medialis narium. The appearance of the cavum medium within the ventrolateral extension of the septum nasi divides the latter into an upper component, the lamina superior (l. sup.) lying between the cavum principale and cavum medium, and a lower part, the lamina inferior (l. inf.) lying ventral to the cavum medium. As the cavum medium increases in width in posterior sections, the lamina superior and lamina inferior lose their lateral connection. The lateral part of the cavum medium diverges in the region of the external nares as the nasolacrimal duct (ncl. dt.) and the cavum medium becomes confluent with the cavum principale ([Fig. 6]). The posterior end of the cavum medium lies at the level of the posterior terminus of the septomaxillary.
Slightly posterior to the anterior end of the cavum medium the foramen for the ramus externus narium (r. ext. n. f.) and ramus medialis narium (r. med. n. f.) opens ventromedially into the floor of the septum nasi ([Fig. 4]). The ventral closure of the floor of the foramen completes the solum nasi, marks the anterior end of the recessus medialis of the cavum inferior, and differentiates the roof of the recess, the crista intermedia (cr. int.), from the solum ([Fig. 5]). The crista intermedia joins the laminae superior and inferior and joins them for a short distance to the septum nasi medially. The anterolateral part of the cavum inferior (cav. inf.) lies ventrolateral to the cavum medium, and extends medially to join the medial recess. The fusion of the two recesses of the cavum inferior completely separates the lamina inferior from the solum nasi ([Fig. 6]).
Near the level of the union of the recessus lateralis and recessus medialis of the cavum inferior, the crista intermedia separates from the septum nasi, and the lamina superior diverges at its mid-width to accommodate the septomaxillary (spmax.) ([Figs. 5] [6]). The lateral remnant of the lamina superior and transition zone between the lamina superior and lamina inferior is short and is quickly replaced by connective tissue. Slightly posterior, the distal edge of the lamina inferior diverges laterally as a small process, which lies dorsal to the pars facialis (p. fac.) of the maxillary. This part of the lamina inferior terminates posteriorly at the level of confluence between the cavum principale and lateral recess of the cavum inferior.
Slightly posterior to the divergence of the crista intermedia from the septum nasi, the crista terminates, thereby separating the lamina superior and the lamina inferior from one another ([Figs. 6] [and 8]). The lamina superior terminates at the level of the infundibulum (inf., [Fig. 7]), whereas the lamina inferior extends posterolaterally, increases greatly in depth and joins the laterally ascending oblique cartilage (obl. c.) to form the planum terminale (pla. ter., [Fig. 10]).
The crista subnasalis (cr. sub., [Fig. 6]) differentiates from the lateral edge of solum nasi adjacent to the maxillary in sections just posterior to the confluence of the recessus medialis and the recessus lateralis. The crista persists as a rod of cartilage which gradually diminishes in size and terminates at the posterior level of the septomaxillary.