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Show 130 ON THE VERTEBRATE SKULL. cribriforn1 plates, leaving the ethmoid prop r, or lamina p erpendicularis, 11pon their inner side, and the lateral mn~ .cs of the ethmoid, or superior and middle spono-y bones, n~on th u: oute.r sides. The optic nerves pa s out through the optic foramina, situated between the roots of the orbito-sphenoids, from the chiasma, which rests, as has just been stated, npon the posterior and upper part of the presphenoid. lienee it fol~ows, that tho presphenoid lies in front of, and between, the opt1c nerves, whiCh embrace it, as in a fork, from behind. rrhe third and fourth pairs are not of SO lUUCh morphological in1portance that I need dwell upon thmn, but the trigen1in al affords first-rate craniallandrnarks by its na al branch and itB whole third division. The nasal nerv enters the orbit by tho foramen lacerum anterius, pa ses to th inner side of the eye, and then, traversing the anterior of the two ethmoidal foramina, perforates the "lateral mass of th ethmoid," and entering the cavity of the bony cranium, though it alwny. lies beneath tho dura mater, skirts the olfactory aperture, and pas "'es out into the nasal cavity, by an aperture in the front part of the cribriform plate. We shall find this in·egular perforation of the "lateral mass of the ethmoid," by the nasal clivi ion of the fifth . nerve, to be an excellent guide to the determination of the homologue of the bone in the lower Vm·tebrata. The third division of the trigeminal traverses the foramen ovale in the posterior part of the alisphenoid, so that it nu1kos its exit behind the greater part of that bone, and altogether in front of the periotic bone. ~rhe portio dura enters the internal auditory foramen in tho periotic mass, runs along its canal, situated above tho f enestrra ovalis, and eventually passes out by the stylo-mastoid foramen. It therefore perforates the fore part of tho periotic·, passing in front of the membranous labyrinth. The portio moll'l's also enters the periotic bone by the internal auditory foramen, and it terminates in the metnbranous labyrinth. The eighth pair passes out through the foramen lacerum posterius completely behind the periotic (which thus lies between the exits of the fifth and of tho eighth pairs), and 1n front of the ex-occipitals. THE STHUCTUHE OF THJ~ HUMAN SKULL. 101 The ninth pair perforates tho ex-occipitals in front of the conflyles. With regard to the relations of the nerves to tho inferior arches of the skull, only one circumstance calls for particular notice,-the distribution of the terminal divisions of the portio d~wa. This nerve divides) aH it is about to leave the temporal bone, into two portions, the larger of which pa~ses out by the stylo-mastoid foramen, aud, besides giving off many other branches, supplies certain muscles of the hyoidean apparatns. The smaller division of the nerve, of comparatively insignificant size-the chorda tympani-returns to the tympanic cavity, crosses it, and leaving it by an aperture internal to, and above the tympanic eleinent, runs down upon the inner side of the lower jaw. In Man, the great development of the facial muscles gives a predominance to the branches of the portio dura which supply them; but, in the lower Vertebrates, the nerve becomes more and more completely represented by simple Inandibular and hyoidean divisions, corresponding respectively with the chorda tympani and the branches distributed to the stylo-hyoid and digastric. In the preceding description of the architecture of the human skull, I have, as far as possible, avoided complicating the generaJ view of its structure which I have desired to give, by enter- Fig. 55. NI .E7~o. Fig-. 55.-Ih~mnu lel't temporal Lone, half the 1wtund ::;ize.- u u, po.stcrior root or the zygomatw ~rocc.s.s; c, midule root; f, anterior root; u, post-auditory to.s.sa; m i long processes ot the malleus HllU or the incus. ' K 2 |