172 



VERTEBRATE SKELETON 



plates which are probably the anterior parts of the marginal taeniae. Behind 

 the optic nerve, and connected with the basal plate, is an ascending cartilage 

 which, as it is perforated by the third, and sometimes by the ophthalmic branch 

 of the fifth nerve, is clearly sphenolateral. Later this grows still farther dorsally 

 and joins the otic capsule behind, this part apparently being the preotic pillar. 

 This posterior part suddenly diverges, enlarging the cavity for the brain, which 

 is made still larger by the lowering of the floor between them to the trabecular 

 level. An interesting feature in Tinnuncidus is the existence of two islands of 

 precartilage in the cranial roof, on the hinder side of the pinealis. 



Fig. 185. — Chondrocranium of chick (Tonkoff's model, in Gaupp, '05). ao, ant- 

 orbital process; as, alisphenoid cartilage;/, frontal;//), preotic foramen; io, interorbital 

 septum; J, jugular foramen; mk, Meckelian; n, nasal; ns, nasal septum; oc, occipital 

 region; p, parietal; pf, prefrontal; ps, parasphenoid; qj, quadratojugal; sq, squamosal; 

 55, supraseptal plate; z, zygomatic. 



The nasal septum is a continuation of the common trabecula or of the 

 intertrabccular cartilage and is continued forwards by the prenasal cartilage. 

 Two fenestras appear in the septum, one soon closing; the posterior, just behind 

 the nasal sacs, enlarges in many birds so that the septum is interrupted except 

 at its base, thus forming a cranio-facial fissure, important in the later mobility 

 of the beak (p. 170), the whole nasal labyrinth participating in the motion. 



