VERTEBRATES. 



17 



ro 10 — H IS, „f rrinium of 

 aM,-lnl)r\..i-ln,k.r.con,Ule, 

 in. p\nci ipit ll I. flolll it, /is, 

 boiizoiit li, mil p^, posterior 

 semicircul.ir raii.tl!>. ;;, pirie- 

 tal . s sphenotic procC'S , w, 

 supraocoipital, sj.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 cletnents into the occijiital bone. The three most 

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

 epiotic above, and the opistliotie beliind, but certain adiiitional elements are found 

 forming part of the wall of the ear-caj)sule as well as the 

 socket for tlie suspensorium in Teleosts, the sj)heiiotic (post 

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

 their 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-s])henoids. In the same relation to these that the 

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

 splienoids, which form part of tlie lateral wall of the cranium 

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

 roof of the chondrocranium, here comjiarable to the su])ra- 

 occijiital, and, indeed, in many forms the basal elements persist only as cartila<'e. 



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



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



the intcrorbital septum — in which the sphenoidal elements arc to be souijht. But the 

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

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

 they are destined. A nasal septum, derived from the coalesced trabecuhe, se|iarates the 

 olfactory organs of the two sides, and is continuous with lateral ])lates jtrotecting the 

 organs. If ossifications appear in these septal and. lateral cartilages, they ai'e 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 turbinal 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, frontals, ])re- 

 frontals, and nasals (Fig. 20). 



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

 paras])henoid behind, retaining in many instances the teeth to which they owe their 

 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 jtarictals, 

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



VOL. III. — 2 



HI, angular ; rtr, articu- 

 f, frontal ; im, prcinax- 



^iUary ; v., nasal ; ;>, parietal ; 



ijj, q'uaiirato-jugal ; a, supra- 



