188 



REPORT — 1846. 



question to be determined, therefore, is, what are the essential characters re- 

 spectively of the 'alisphenoid' and the 'petrosal' in the vertebrate series? 



Those, of the alisphenoid appear to me to be the following : — 1st, its con- 

 nection below with the basisphenoid and behind with the petrosal, where it 

 forms the forepart of the ' otocrane' or cavity for the reception of that osseous 

 or cartilaginous immediate capsule of the labyrinth or internal organ of hear- 

 ing: the alisphenoid is also commonly, but not constantly, joined before 

 with the orbitosphenoid, and above with the parietal: it has other less con- 

 stant connections with the squamosal, the exoccipital, the supraoccipital and 

 the basioccipital : 2ndly, with regard to its essential functions, the alisphenoid 

 protects more or less of the side of the mesencephalon, or (in mammals) of 

 the middle lobe of the hemisphere : it gives exit, by notches or foramina, to 

 the third, and usually, also, to the second divisions of the trigeminal or fifth 

 pair of nerves. 



The essential character of the petrosal is to envelope immediately the 

 whole of the vascular and nervous tunics of the labyrinth or internal organs 

 of hearing, either in a membranous, a cartilaginous or an osseous state ; 

 its histological condition being much less constant than that of the alisphe- 

 noid. 



On viewing the alisphenoid on the interior surface of the human skull 

 (fig. 6, e), it seems to be the least significant and important part of the lateral 



Fig. 6. 



Vertical longitudinal section of the human cranium. 



walls of the cranial cavity : it forms their smallest portion : it is much sur- 

 passed in extent by the squamosal (ib. 27) and the supra-occipital (ib. 3), 

 and still more so by the enormously expanded parietal (7) and frontal (11). 

 Nevertheless we find it connected, anchylosed indeed, below to the basisphe- 

 noid (5), bounding anteriorly the space into which the petrosal (16) is 

 wedged ; connected in front with the orbito-sphenoid (10), and usually 

 articulating by its superior apex with the parietal : I purposely omit the 

 mention of other connections of the alisphenoid in Man which are less 

 constant in the vertebrate series. But it is important to observe, notwith- 

 standing the displacement which the alisphenoid has undergone through the 

 intercalation of the extraordinarily developed squamosal into the lateral walls 



