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Show 228 ON 'l'HE S'l'RUCTURE OF THE EKULJJ. f :F. hes and therefore the line of be homologous with those o 1JS f tals of the Pike with argument which identified theb pr~dro~ the M'tn is equally 1 of the et mm 111 (. the latera masses . R til s In Birds the post-licable to the same bones In ep e . . . ' farpopn taJs have on 1Y a d ou btful and exceptional d11st 1nctne'stsh, atnl d in them the t rue pre fI.O n tals seem. early to c.o a esce WI '·fi 1de h 'd The last-mentioned cranial element IS usually ossi c et mo1 . c; 11 · B' 1 and appears upon the upper surface of .t he sku'l ' .I n In Is ; whi.l e, I.n R ep t~ ·z~·a , it almost alw.a v" s remains carti ag'idno 1u s. · n the exti.n ct D w. yno d0 n ' however ' It and the presphenoi a regwn were completely ossified. In Birds, in consequence of the prolongation of the snout Fig. 92. A Pt 2~ Fig. 92.-Lateral views of the skulls of (A) an Ostn.c h, (B·)u a Chr oc~sd~'l eb eaennd r(eCm)o vae Pdy. tho1'n1,, without the mandible. In the Python's skull the maxi a as a turbinal bone of the Ophidian. 1'HE SR UJ.;LS OF REP1'ILIA AND AVES. 229 into a beak, the internasal part of the basi-facial axis acquires a considerable size, aud becomes the subject of a great variety of ossifications, which, in many Birds, are so arranged as to allow the anterior part of the cranio-facial axis to be moveable on its posterior part. In many Lizards, on the other hand, the anterior part of the cranium is rendered moveable upon the posterior in another way. The cranio-facial axis in front of the basisphenoid is cartilaginous, and consequently slightly flexible, w bile the roof of the skull between the parietals, the supraoccipital and the periotic bones is merely membranous (t, Fig. 90, A) ; hence, the front part of the skull is capable of being slightly raised or depressed, in a vertical plane, upon the posterior part. Next to the pro-otic, the squamosal and the quadrate bones of Birds and Reptiles have been the subject of the greatest amount of controversy among morphologists. The bone which was originally called "os quadratum" is that moveable facial bone of the Bird ( Qu, Fig. 92, A) which is articulated, above, with the outer side of the periotic capsule, and especially with the pro-otic bone, and below with the os articulare of the mandible, while, internally and anteriorly, it is connected with the pterygoid. In the Crocodilia (Fig. 92, B) and Chelonia, a bone, admitted by all to be the homologue of this, is attached immoveably in the same region: in most Lacertilia (Fig. 93) it is moveable, and remains connected with the produced extremity of the pro-otic bone; but, in most Ophidia (Fig. 92, C) its proximal end is thrust out from the skull upon the extremity of another bone. However, its homology with the quadrate of the Bird is not affected by this circumstance. With what bone in the human skull does this correspond? Ouvier identified it with the tympanic of Man, and his interpretation has been generally accepted; but the tympanic is always a membrane bone, whereas this is always a cartilage bone. The tympanic directly supports the tympanic membrane, while this bone sometimes gives no direct attachment to the tympanic membrane at all. The tympanic of Mammals again becomes smallest in those Mammalia which most nearly approach Birds |
