VERTEBRATES. 



IT 



Fig. 19. — Base of cranium of 

 an embrjro chick ; c, condyle; 

 eo, exoccipital;/, frontal; hs, 

 horizontal, and p.i, posterior 

 semicircular canals; p, parie- 

 tal ; 5, sphenotic process ; so^ 

 supraoccipital; $(/, squamosal. 



in lower forms, but in the adults of higher forms they fuse on each side into one mass, the 

 petrosal bone, as do the four occipital elements into the occipital bone. The three most 

 commonly occurring elements are the prootic in the fore-part of the ear-capsule, the 

 epiotic above, and the opisthotic behind, but certain additional elements are found 

 forming part of the wall of the ear-capsule as well as the 

 socket for the suspensorium in Teleosts, the sphenotic (post 

 frontal) and pterotic. These bones may be partly dermal in 

 tlieir origin ; the latter being intimately connected with a 

 dermal bone, the squamosal. 



In the cartilage of the base of the skull, between and in 

 front of the auditory capsules, one or two bones appear, con- 

 tinuing forward the basioccipital ; these are called the basi- 

 and pre-sphenoids. In the same relation to these that the 

 exoccipitals have to the basioccipital are the ali- and orbito- 

 sphenoids, which form part of the lateral wall of the cranium 

 in the orbital region. But there is no ossification in the 

 roof of the chondrocranium, here comparable to the supra- 

 occipital, and, indeed, in many forms the basal elements persist only as cartilage. 



In many forms, as birds, the cranial cavity is very much compressed from side to 

 side in the orbital region. The result is a mere thin partition between the orbits — 

 the interorbital septum — in which the sphenoidal elements are to be sought. But the 

 olfactory region is either entirely in front of the cranial cavity, or two tubular prolonga- 

 tions of the latter convey forwards the olfactory tracts to the neuro-epithelium to which 

 they are destined. A nasal septum, derived from the coalesced trabeculae, sepai-ates the 

 olfactory organs of the two sides, and is continuous with lateral plates protecting the 

 organs. If ossifications appear in these septal and lateral cartilages, they are termed 

 mesethmoid, and lateral ethmoids. When the surface on which the nasal mucous mem- 

 brane is distributed becomes 

 very complex, as it does in 

 mammals, special ingrowths 

 of the olfactory capsule ossify 

 as tnrbinal bones. 



Related to the roof of the 

 chondrocranium are mem- 

 brane bones which, when the 

 cartilaginous box becomes 

 insufficient in size to accom- 

 modate the brain, fill up the 

 gaps, and become thus di- 

 rectly related to the brain. 

 These are known as parietals, 

 squamosals, f rentals, p r e - 

 frontals, and nasals (Fig. 20). 



Similarly related to the ventral face of the chondrocranium are the vomer in front, and 

 parasphenoid behind, retaining in many instances the teeth to which they owe then- 

 origin. All of these membrane bones persist in the higher classes of vertebrates, some 

 of them, such as the parasphenoid, losing, while others, like the frontals and parietals, 

 gain in importance. But we may observe in the Teleosts many membrane bones 



VOL. III. — 2 



FlO. 20.— Skull of young sage-cook {Cenirocercus); an, angular ; ar, articu- 

 lar ; as, alisphenold ; d, dentary; e, exoccipital ; /, frontal ; im, preiuax- 

 illary; ?, jugal ; me, mesethmoid ; mx, maxillary ; n, nasal ; p, parietal ; 

 jD/, palatine ; i)f, pterygoid ; g, quadrate; gj, quadrato-jugal ; s, supra- 

 occipital ; san, surangular ; sg, squamosal. 



