| OCR Text |
Show 300 ON THE S'l'llUUTUHE OF THE VEH'fEBRATE SKULL. cartilaginous plat , which results from the coalescence and outgrowth, beyond the boundaries of the brain-case, _of the tr~bec~lte cranii; partly, by outgrowths fron1 the supenor. and Infenor edges of that plate ; and partly, by a prolongation outwards of the front part of the outer wall of the brain-ease, into au antorbital process, between the orbit and the nasal sac, on each side. Cartilage bone developed in the septum gives rise to the eth1noid ; in the antorbit~l processes, to the prefrontals ; in the superior, or inferior, lateral prolongations of the side walls, to the turbinal bones. Membrane bones developed upon the roof of these olfactory capsules give rise to nasals; beneath the septum, to vomers. 13. The ethmoid and its dependencies are developed within the median "fronto-nasal" process, which grows out from tho front wall of the embryonic skull, between the rudimentary nasal sacs; and the inferior, broad, free edge of which bounds the mouth. It is in this free edge that the premaxilloo arc developed, and they are, at first, perfectly distinct from the maxilloo. The latter, together with the palatine and pterygoid bones, are formed within the maxillary processes, which bound the sides of the primitive oral cavity, and run, parallel with one another, along the base of the fore-part of the embryonic cranium, uniting, behind, with the first visceral arch, but being, at £rst, completely separated, anteriorly, from the fronto-nasal process. Clearly therefore, if the premaxilloo and maxillm, &c., are to be regarded as constituents of inferior arches of the skull, they are not parts of one arch, but of, at least, two distinct arches. 14. Of the first and second visceral arches, which lie immediately behind the mouth, the former, which gives rise to the mandible and quadrate bone, passes into the skull under the front part of the auditory capsule; while the root of the latter, in which a greater or smaller part of the hyoidean apparatus is developed, underlies the hinder part of that capsule. It is therefore impossible that the mandibular and hyoidean arches should be dependencies of any other parts of the skull than those which lie imn1ediately in front of, or behind, the auditory capsules ; and in the completely ossified skull we never, as a matter of fact, meet with these arches in any other position. 'l'HE TllEOUY OF THE VliJR'fEBRATE SKULl,, 301 1.5. There is not a shadow of evidence that tllc 111() l'b ] d h 'd · c . "~nc 1 n ar . an ym ea.n arches suffer any shifting of p osi' tI' on f rom b "' 1ore backwards, In the eourse of their development . b t th 't' f h , u e extre-mi Ies o t ose arches which are attached to th IT 11 d • c · e s \.U un ergo very Singular metamorphoses, the effect. of which is tl t th d t r t f h . . ' 1a e ~n ar) pai o t e ~audible Is brought into closer connection with the skull the h1aher we ascend in they t b t · 11] • • • • o er e ra e senes. 1US, In the Fish It lS separated from the skull b th l d'b 1 • . Y e 1yoman- I u ai' quadrate,. and artwular bones ; in the Reptile L the quadrate and articular; while in the Mammal tl yd d h . · 1e qua rate an t e artiCular are metamorphosed into tl 1e I·n cus an d t h e m'atlhle tuhs , and the dentary comes close to the skull t' 1 t' . WI e squamosa1. , ar Icu a Ing These are, I believe, the most important facts regarding the structure and development of the skull, which n1ay now be regarded ~s well establis~ed .. If we inquire how they bear upon .theones of th~ skull, It will be obvious that they place tho doctnne of the unity of organization of the vertebrate skuJJ upo~ a perfectly sure a~d stable footing, while they appear to me, as clearly, to negative the hypothesis that the skull is in any sense, a modification of verteLrm. ' . But though the skull has not a vertebral structure, and in ~ts Inembranous and c~rtilaginous states is not even segmented, It assumes a very definite segmentation in its completely ossified state. In every well-ossified cranium there is, assuredly, an occipital segrne~t .C~' ~ar VeTtebra" of ?~en), formed by the basi-occipital, ex-ocmpi\~ls, and supra-ocC1pital; a parietal segment ("Jaw V ~rtebra of ?ken), constituted by the basi-sphenoid, a]isphenmd, and panetals ; a frontal segment ("Eye Vertebra " of Oken), composed of the presphenoid, orbito-sphenoids and frontals; and a nasal segment ("Nasal Vertebra" of Oken) formed by the eth1noid, prefrontals, turbinals, nasals and vonaer. ' Leave out the hypothetical considerations that these segments ar~ equivalent to one another, and that they are hmnologous With vertebrm, and Oken's expression of the broad facts |
