The Skull as a Whole. 



83 



piriform aperture (apertura piriformis). The cavity is divided into right 

 and left portions, the nasal fossae. In the divided skull it is seen that 

 the division is effected chiefly through a median vertical, cartilaginous 

 plate, the nasal septum (septum nasi), or cartilaginous portion of the 

 mesethmoid. This is continuous posteriorly with a small crescentic 

 vertical plate of bone, the perpendicular plate (lamina perpendicularis) of 

 the ethmoid bone — the bony portion of the mesethmoid — and the latter 

 is also the terminal element of the series of median bones constituting 

 the basicranium. Posteriorly, the ventral portion of the cartilaginous 

 nasal septum is supported by a vertical bony plate, the vomer, the 

 dorsal margin of which is grooved to receive it. Anteriorly, the nasal 

 septum bears on its ventral margin the paired enclosures of the vomero- 

 nasal organ, which are also supported by the grooved surface formed in 

 the middle line by the adjacent dorsal surfaces of the palatine processes 

 of the premaxilla. The relations of these structures, as well as of the 

 cartilage supporting the nasopalatine duct, are best seen in very young 

 animals (cf. Plate III). 



The delicate, folded, or scroll-like turbinated bones, characteristic of 

 the nasal cavity, are borne on its posterior and lateral walls. Occupying 



Fig. 33. The skull in vertical section: BO, basioccipital (basilar portion of occipital); 

 BS, basisphenoid (body of posterior sphenoid); ET, ethmoturbinal ; F, frontal; I, inter- 

 parietal; M, maxilla; MT, maxilloturbinal ; N, nasal; NT. nasoturbinal; P, parietal: 

 PL, palatine; PMX, premaxilla; PR, presphenoid (body of anterior sphenoid) ; PT, petrous 

 portion of petromastoid; SO, supraoccipital (squamous portion of occipital) ; T, tympanic; 

 V, vomer. 



a.p., piriform aperture of nose; c.f., internal aperture of facial canal; c.o., occipital 

 condyle; f.c.a., f.c.m. and f.c.p., anterior, middle, and posterior cranial fossae; f.f., floccular 

 fossa; f.h., hypophysial fossa; f. in., incisive foramen; f.s., sphenopalatine foramen; 1., per- 

 pendicular plate of the ethmoid; m.a.i., internal acoustic meatus; o., optic foramen; p. a., 

 alveolar process of maxilla; p.d., hard palate; p.o.e., external occipital protuberance; 

 p.pt., pterygoid process of posterior sphenoid; s.n., nasal septum; t.c, tentorium cerebelli. 



the anterior portion of the lateral wall of the nasal fossa is a finely-ridged 

 mass of bone, the concha inferior, or maxilloturbinal. It is easily dis- 

 tinguishable from a more dorsal and posterior series of broader folds, 

 which together constitute the ethmoturbinal. In the rabbit, as in mammals 

 generally, the latter is divisible into a more dorsal elongated portion 

 attached to the nasal bone, the nasoturbinal, and a more ventral portion, 

 also posterior portion, the ethmoturbinal proper, composed of several 



