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Show 298 ON THE STHUOTURE OF THE VERTEBRA'rl!J SKULL. on the speci·a l characters of a s1 T~ u 11 , and the oth. er those of a ver- tebral co 1u mn; th e 1a tte i. tac king one road• ' while the skull takes another. The skull is no 1nore a modified vertebral column thau the vertebral column is a modified skull; but the two are essentially separate and distinct modifications of one and the same structure, the primitive groove. . . . . 6. The skull, having assun1ed its special and d1stmctive characters, may pass through three successive ~tates-the membranous, the cartilaginous, and the osseous-In the course of its development; and the order in which these states succeed one another is always the same, so that the osseous skull has a cartilaginous, and the cartilaginous, a Inembranous, predecessor. Nor does any one of these .states ever completely aLliterate its predecessor; more or less cartilage and Inembrane enterino- into the composition of the most completely ossified skull, ~nd more or less membrane being discoverable in the most completely chondrified skull. 7. The adult skull may, however, have got no fui~ther tha~1 one of these states. In the Amphioxus, the skull ( 1f skull It can be called) is membranous. In many Fishes: as we h~ve seen, it is cartilaginous, with, at most, a superfi~Ial conversiOn into bone. In the rest of the Vertebrata definite bones are added, to the more or less complete exclusion of the cartilaginous cranium. . . 8. \Vhen definite cranial bones are developed, they anse In one of two ways, either in the substance of the carti~aginous cranium, as "cartilage bones," or in the perichondnum, o;: remains of the membranous cranium, as "membrane bones. . It is highly probable that, throughout the vertebra.te serie.s, certain bones are always, in origin, cartilage bones, while certam others are always) in origin, membrane bones. 9. With the exception of Amphioxus, three sets of ~ensory organs-olfactory, optic, and auditory-are evolved In the walls of the skull of every vertebrate animal, and they are disposed, from before backwards, in the order in which they a:·e nan1ed. All these sensory organs are originally develo~ed Ill connection with involutions of the integument, which, In the case of the olfactory organ, remain open, but, in tha.t of the eye 'l'HE THEORY OF THE VERTEBHATE SKULL. 299 and ear, become ~hut. Each sensory apparatus is, throughout the Vertebrate senes, related to the same nerves . th 1~ t b · 1· . · e o 1ac ory mn. g supp wd by the . first p.a ir·, the optic, by th e seconc1 ; th e au?Itory, by the portw molb,s of the seventh; while the fifth p~Ir leav~s the. sku!l inr front of the auditory capsule, and the eighth pair behind It. These relations of the cranial nerves to the sensory organs, and consequently to the ci·an1'al 11 . bl. 1 d wa s, a 1 e esta. Is 1. e antecedently to chondrificatio1J , and a' .I1 '.o r tw · r2· t o ossification ; so that the cranial nerves aud the sensory organs se. rve as fixed points by which the nature of the v an·o us ossi'f i caN tions can be determined. ~0. By t~e help of these landmarks, chiefly, it has been pos~I~le to Identif! . the bo~es known as basi-occipital, ex-ocCJpitals, supra-ocCipital · basi-sphenoid alispheno'd · t 1 • • ' • , 1 s, pane a s ; presp.henoicl, orbito-sphenmds, frontals ; or, in other words, the co~stituents of the ':alis of the brain-case, throughout the whole senes-from the Pike to Man. And it is found that these bones, w~e? they ~11. occur together, are so disposed as to form three, ongmally diStinct, segments. 11. Recourse to long-established, but frequently-foraotten facts i~, the ~~story of th~ de~elopment of the so-called ~'pars petrosa, and pars masto2dea, or periotic bone, of the human sln:ll, has sho~n that these parts ossify from three centres, whiCh have hitherto received no names, and which I have termed the "pro-otic," "opisthotic," and "epiotic" bones. It has been one of the principal objects I have had in view to prove, by paying careful attention to the relations of these osseous elmnents, on the one hand to the nerves, and on the other to the parts of the auditory organ which they enclose, t~at they are very generally represented, sometimes in a distinct form, and sometimes coalesced with one another, or with o.ther bones, throughout the series of skulls provided with cartilage bones; and that the pro-otic, especially, is one of the most constant and easily-identifiable bones throughout the series of vertebrate skulls. J2 · Tl1 e eye I·s not I· nvested by any cartilaginous or osseous elements of the cranial wall ; but the olfactory sacs become Ino·r e or 1e ss enc1 o se d I·n a capsule, fonned partly by a n1edian |