SKULL — BIRDS 



175 



M' 



omx 



between basicranial and hypophysial fenestra?, the dorsum sellae 

 cannot ossify in the usual manner, but is formed by growth from 

 below. The ahsphenoids are associated with the basisphenoid, ossi- 

 fying in the preotic pillars by two or three 

 centres in each. These bones, which ossify 

 late, form the greater part of the posterior 

 wall of the orbit and largely close the 

 brain cavity laterally and in front. The 

 second and third rami of the fifth nerve 

 make their exit between the alisphenoid 

 and the otic capsule, passing either 

 together or through separate foramina. 

 The aHsphenoid fills the gap between 

 orbitosphenoid, interorbital septum, 

 petrosal, frontal and squamosal, the latter 

 usually separating it from the parietal. 

 The presphenoid has two centres in the 

 base of the interorbital septum, the result- 

 ing bone being small and fusing with the 

 adjacent bones, it and the underlying 

 part of the parasphenoid uniting as the 

 sphenoidal rostrum. The orbitosphenoid 

 ossifies from a centre in the supraseptal 

 plate, and extends downwards, the result- 

 ing bone being small. Between it and 

 the alisphenoid are the exits of nerves II, 

 III, IV, and VI, and the ophthalmic 

 branch of V, some of which may have 

 separate foramina in the adult, some 

 passing out with the optic nerve through 

 the orbital fissure. When a separate 

 optic foramen occurs, it is near the base 

 of the bone. 



The olfactory nerve leaves the main 

 cranial cavity at the junction of frontals and orbitosphenoid, runs 

 forwards in the groove between the supraseptal plates to the ethmoid 

 region. There are several centres in the nasal septum and wall of 

 the nasal capsule, and a separate centre in the antorbital plate 

 These form a mesethmoid and a pair of ectethmoids which soon 



Fig. 189. — Palate of duck. 

 bt, basiteniporal plate; ch, 

 choana; eo, exoccipital; j, 

 zygomatic; mx, maxilla; p, 

 palatine; pmx, premaxilla; pi, 

 pterygoid; q, quadrate; qj, 

 quadratojugal; r, rostrum; v, 

 vomer. 



