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Show 142 ON THE STRUOTUHE OF 'l'liE SKULh I f. t the coalesced trabecu lro become changed into a plat n ron ' . 'd 1 . 1 . . h of car t 1·1 age, compres· sed fr01n 1. de to s1 , w n 1 occupies t e Iniddle of the gradually-narrowing fronto-nasal proc ss, as the tl1mo-vomerine or internasal, cartilage. e From the sid' es of the basi-sphenoid cart1' la gu. 1ous p 1a tes arc developed, which foreshadow the form an~ relatio:1s of tho alisphenoids; at the sides (Jf the presph no1:lal reg1o~ of tho cartilagE\ similar plates represen~ the orb1to- phen~1ds. In front of these the upper part f the 1nt rna al, or tlnno1dal, cartilage passes laterally into broad deflected cartilagiuous_ lainellro, which curve round the olfactory sacs, and occupy tho places of the lateral masses of the ethmoid and the in£ rior turbinal bones. Thus far the terms of my description are almost as applicable to the embryonic cranimn of l\fan a to that of the chick. The human cranium has been observ d forming part of an open grooYe ; it undergoes a flexure, and develops visceral arches altogether similar to those of the chick, nor is there any reason to doubt that the organs of ense are <Ieveloped in the same manner. The very earliest condition of the cartilaginous cranium of the human embryo has not been observed; but, at the beginning of the second month, it consi t wholly of cartilage and of membrane, disposed in a 1nanner which differs only in detail from that seen in the chick. Thus the occipital foramen is surrounded by cartilag , continuous with that which extends through the basi-sphenoidal, pr phenoidal, and ethmoidal regions to the anterior end of the face. The alisphenoids and orbito-sphenoids are repr ent d by cartilage, and cartilaginous · plates arch down from the summit of the internasal cartilage, on each side, to form the substratum on which, the nasal bones and, in which, the spongy bones will be d v loped. That part of the cranial cartilage which lodges the auditory organ is exceedingly large, and constitutes, not only an oval cflpsule for the membranous labyrinth, but sends back a continuation which fills the space corresponding to the pars mastoidea, and extends somewhat higher than it beneath the parietal region of the skull. All the upper part of the cra11ium is and remains simply n1em branous. THE DEVELOPMENT OF THE HUMAN SKULL. 14:i The relations of the regions of the chondro-cranium thus formed to the parts of the brain and to the exits of the nerves are the same as those which are observed in the bones which they prefigure. When these bones begin to be developed, some of tho1n make their appearance in the cartilage of the embryonic skull, some in the perichondrium, others in the membranous roof which is continuous with the perichondrium. A single ossification appears around the notochord in the basi-oecipital regjon, and lays the foundation for the basioccipital bone. The ex-occipitals com1nence as single centres of ossification in the cartilage bounding the sides of the foramen magnum. The supra-occipital (SO) is developed from two ossifications in the cartilage above the foramen magnum, and b a. 1 z II E Fig. 58.-Successive embryonic conditions of the human head (after Ecker).-I, I£, the first and second cerebral vesicles. 1, 2, 3, 4, the visceral arches; a, the maxillarv P1:oces!';; b, the eye; nf, the middle naso-frontal process ; c, the lateral naso-fron~l P~ ocess.; t, the tongue; au,, the outer part of the first visceral cleft, which e\'entually g1ves nse to the external auditory meatus. A, at three weeks. B, at five weeks. C, at r-;ix weeks. D, at se\·en weeks. E, at eight weeks. F, outline side view of K |
