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Show 222 ON THE S'l'RUOTURE OF THE SKULL. and Reptiles the relations of this bone are e~sentially such as arc shown in the accompanying :figures of sectiOns of the skulls of an Ostrich, a Crocodile, and a Turtle (Fig. 88, A, B, C). In all these it will be observed, that the aperture for the exit of the third division of the trigeminal ( V) lies in front of a bone, ·which is notched, or perforated, by apertures for the portio dura and portio mollis, and that the anterior part of the organ of hearing is lodged within this bone. Furthermore, an external view of this region of the skull (Fig. 89, A and B) shows that the bone in question contributes, in each case, the anterior half of the boundary of the fenestra ovalis. In other words, the bone in question has every essential relation of that ossification which, in Man and in the Pike, I have termed pro-otic.* The other elements of the periotic capsule are not far to seek. In the 'furtle one of them retains its independence throughout life, and occupies a considerable space on the exterior of the skull, though, internally, only a small strip of it is seen in front of the foramen for the eighth pair (Fig. 88, C). This bone furnishes the posterior half of the frame for the fenestra ovalis, with so n1uch of that of the fenestra rotunda as is osseous, and it lodges the posterior and outer part of the auditory organ. It answers precisely, therefore, to the opisthotic. t The corresponding ossification in most other Reptiles and in Birds early coalesces with the ex-occipital. The third periotic bone, the epiotic, does not remain distinct throughout life in any Reptile or Bird, and its place appears to be taken by a triangular process of the supra-occipital, which * This is the bone called by Cuvior " rocher," and regarded by him and by most of the German anatomists as the homologue of the pars pet'rosa of the ]mm:m temporal bone. I took the same view myself when I d livered the Crooninn Lecture in 1858, and I do not now substantially depart from it. For that part of the pars petrosa which is most obvious and visible in the interior of the &kuU is its pro-otic portion ; and so long as the complex nature of tho pars petrosa was unknown, the identification of the bono Pr.O in the Bird and Reptile with the petrosal of the Mammal was the nearest approximation that could be made to the truth. Cuvier's identification would have been absolutely correct if be had termed the ornithic and reptilian bone not "petrosal," but "anterior part of the petrosal." t Cuvier termed this bone the " occipital externe." Hallmann regarded it as the equivalent of the "mastoid," and I followed him in my Croonian Lecture. In the absence of a full knowledge of the development of the human pars petrosa, it was difficult, if not impossible, to see one's way to any better conclusion. THE SKULLS OF REPTILIA AND AVES. 223 shelters the summits of the vertical . . I 1 semicucu ar canals But t 1e. s~udy. o~ development has shown that this part of th~ su ra ocCipitalis, In many, if not all, Reptiles and Birds, de ve I ope d fr~o 1n.. Fig. 89: A F' :f.r. 1g. 89.-External view of the aud · t . and (B) a Turtle ( Chelon; mld~~S reg~o~1 oft~~ skull in (A) a c.rocodile (C. biporcatus) away in each case so far as is . e wa s of the tympamc cavity have been cut J:urtle's skull, the semici;·cular I~c~slsary tol show ~he au_ditory f enestt·m; and, in the skull (A) F.O. is the fen t aUct.s are a so partially dtsplayed. In the Crocodile' thotic, c, fi:om the fenest:~ ~~t~~~~s: separ~ted by the cochlear process of the opis~ (b) of the opisthotic which ~' Chl Is the hook formed by thf' cuiTed process ' suppol ts the cochlea externally. The lower end of the |