118 ME. W. K. PAEKEE ON THE STEUCTUEE AND 



The alisphenoid (Plate VII. figs. 1, 2 & 3, a.s.) is completely preformed in cartilage ; 

 and this cartilage has no membranous fontanelle, or remnant of the original membranous 

 brain-sac ; it is subpentagonal, is notched at its postero-inferior angle for the large tri- 

 geminal nerve (Plate VII. figs. 1, 2 & 3, 5), and is continuous with the periotic capsule 

 behind, and with the sellar region of the basisphenoid below. It is convex outside, 

 and concave within, and is of considerable thickness. 



In the Ostrich-tribe and in all embryo-birds, the posterior sphenoid is further com- 

 plicated by two thick outgrowths of cartilage, the anterior pterygoid processes (Plate 

 VII. figs. 2 & 4, a.p.) ; they arise in the Struthionidse on each side, and somewhat in 

 front of the sella turcica, and project outwards and forwards to abut against the true, 

 or internal pterygoid bones. In this stage (Struthio, A.) they are only slightly ossified 

 at their base by the long inferior bony piece, which is already spreading out poste- 

 riorly. 



The internal carotid arteries (Plate VII. figs. 2 & 4, i.e.) pierce the thick, wide 

 investing mass opposite the anterior end of the basioccipital ; close below each carotid 

 canal, near the edge of the chondrified mass, there is already a small irregular patch of 

 bone; this is the " basitemporal " (Plate VII. fig. 4, b.t.), or "lingula sphenoidalis." 

 Between the coalesced rafters of the skull and the secondary skull-balk (Plate VII. 

 fig. 2, jp.s.-pe.-r.b.s.) (rostrum of basisphenoid) there is much fibrous tissue, except as we 

 approach the anterior cltnoid wall : this fissure is only slowly obliterated. 



The coalesced trabeculee maintain a pretty equal thickness to their termination in front ; 

 near the end, however, they lose their height, become broader, curve slightly downwards, 

 and end in a rounded subspatulate manner (Plate VII. figs. 1-5, ja.n.). After the basis 

 cranii had recovered from its mesocephalic flexure, both the optic and the olfactory 

 sacs approximated ; and the simple, indiff'erent tissue between them became converted 

 into one continuous vertical plate of cartilage with its investing perichondrium. Thus 

 the presphenoid, the vertical, or middle ethmoid, and the nasal septum, form one 

 continuous plate (Plate VII. fig. 2), somewhat thinner between the eyes and the turbinal 

 coils than at the other parts, but altogether thinner than the base of the plate which 

 was formed by the coalescence and chondrification of the trabeculse. 



The highest .part of this orbito-nasal septum is the homologue of the "crista galli" 

 (Plate VII. figs. 1, 2 & 3, cr.g.) of the human skull, on each side of which the olfactory 

 lobes (1) pass to terminate inside the simple rudiments of the cribriform plate {eth., al.e.). 

 The hemispheres of the cerebrum only reach this point, and they lie obliquely on a pair 

 of laminae which grow out of the posterior half of the orbital septum. These oblong 

 plates have their free angles rounded ; each posterior angle lying a short distance from 

 the alisphenoid, and each plate being less than half the size of the posterior sphenoidal 

 ala — these are the orbito-sphenoids (Plate VII. figs. 1 & 2, o.s.). 



In front of these orbital alse the septum is notched above, and this notch passes on 

 each side into the groove for the olfactory lobe ; above these grooves the septum gives 

 out its ethmoidal alse (Plate VII. figs. 1 & 2) : the delicate cartilages of the nasal sacs 



