[Illustration: 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.