THE THEORY OF THE VERTEBRATE SKULL. 299 



and ear, become shut. Each sensory apparatus is, throughout 

 the Vertebrate series, related to the same nerves : the olfactory 

 being supplied by the first pair ; the optic, by the second ; the 

 auditory, by the portio mollis of the seventh ; while the fifth 

 pair leaves the skull in front of the auditory capsule, and the 

 eighth pair behind it. These relations of the cranial nerves to 

 the sensory organs, and consequently to the cranial walls, are 

 established antecedently to chondrification, and a fortiori to 

 ossification ; so that the cranial nerves and the sensory organs 

 serve as fixed points by which the nature of the various ossifica- 

 tions can be determined. 



10. By the help of these landmarks, chiefly, it has been 

 possible to identify the bones known as basi-occipital, ex- 

 occipitals, supra-occipital ; basi-sphenoid, alisphenoids, parietals ; 

 presphenoid, orbito-sphenoids, frontals ; or, in other words, the 

 constituents of the walls of the brain-case, throughout the whole 

 series — from the Pike to Man. And it is found that these 

 bones, when they all occur together, are so disposed as to form 

 three, originally distinct, segments. 



11. Eecourse to long-established, but frequently-forgotten 

 facts in the history of the development of the so-called "pars 

 petrosa" and "pars mastoidea," or periotic bone, of the human 

 skull, has shown that these parts ossify from three centres, 

 which have hitherto received no names, and which I have 

 termed the " pro-otic," " opisthotic," and " epiotic ' : bones. It 

 has been one of the principal objects I have had in view to 

 prove, by paying careful attention to the relations of these 

 osseous elements, on the one hand to the nerves, and on the 

 other to the parts of the auditory organ which they enclose, 

 that they are very generally represented, sometimes in a dis- 

 tinct form, and sometimes coalesced with one another, or with 

 other bones, throughout the series of skulls provided with car- 

 tilage bones; and that the pro-otic, especially, is one of the 

 most constant and easily-identifiable bones throughout the series 

 of vertebrate skulls. 



12. The eye is not invested by any cartilaginous or osseous 

 elements of the cranial wall ; but the olfactorv sacs become 

 more or less enclosed in a capsule, formed partly by a median 



