222 ON THE STRUCTURE OF THE SKULL. 



and Reptiles the relations of this bone are essentially such as are 

 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 Turtle 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 much 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.\ 



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 Cuvier " rocher," and regarded by him and by 

 most of the German anatomists as the homologue of the p> ars petrosa of the human 

 temporal bone. I took the same view myself when I delivered the Croonian 

 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 skull is its 

 pro-otic portion ; and so long as the complex nature of the pars petrosa was un- 

 known, the identification of the bone 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 he 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 jjars petrosa, it was 

 difficult, if not impossible, to see one's way to any better conclusion. 



