172 ON THE STRUCTURE OF THE SKULL. 



they are as clearly and distinctly identifiable as any bones of 

 the skull. 



The large bone, Pr.O., which occupies a great part of the 

 wall of the cranial cavity, in front of the ex-occipital, has its 

 front margin deeply notched for the exit of the third division 

 of the trigeminal. It presents a foramen through which the 

 portio dura passes ; it protects the anterior part of the vestibule 

 and the anterior vertical semicircular canal. In other words, 

 it has exactly those relations to the auditory organ and to the 

 cranial nerves which especially characterise the pro-otic ossi- 

 fication of Man — which, it will be recollected, also protects the 

 anterior part of the organ of hearing, lies behind the exit of the 

 third division of the trigeminal, and is perforated by the portio 

 dura. 



In minor respects, on the other hand, the pro-otic of the 

 Pike differs from that of Man ; as, for example, in its vast pro- 

 portional size ; in its remaining distinct from the other periotic 

 bones ; in the wide ossification which extends from it over the 

 basal cartilage of the skull towards the middle line ; and in 

 coming into contact with the ex-occipital and basi-occipital 

 behind. But none of these modifications really interfere with 

 the homology of the bone — which we shall find to be identi- 

 fiable by the same essential characters throughout the vertebrate 

 series. 



The epiotic element is not less distinctly recognisable. The 

 upper and lower crura of the great posterior vertical semi- 

 circular canal traverse notches in the supra-occipital and ex- 

 occipital respectively, but the summit of the arch of the canal 

 is inclosed within a distinct conical ossification, the " external 

 occipital " of Cuvier. This ossification is, in fact, perched upon 

 the posterior vertical semicircular canal, just as the human 

 epiotic ossification is perched upon the summit of the arch of 

 the same canal when it first appears ; and if the semicircular 

 canals of man were to grow in the same proportion to the brain- 

 case as those of the fish, the epiotic would be carried out as far, 

 and would .leave a considerable space between the pro-otic and 

 itself, into which the adjacent supra-occipital and ex-occipital 

 bones might intrude, as they do in the fish. 



