122 ON THE VERTEBRATE SKULL. 



obliquely from this spot, downwards and forwards, to the 

 point e (Fig. 53) on the uncler-surface of the axis, where its car- 

 tilage becomes continuous with the osseocartilaginous inter- 

 nasal septum. 



It is this osseo-cartilaginous septum between the two nasal 

 cavities, the upper free edge of which constitutes the crista 

 galli, while the lower free edge supports the septum nariwm, 

 which terminates the hasi-facial axis. 



All the upper and middle part of this septum is formed by 

 a thin osseous plate, the lamina perjjenclicularis of human 

 anatomy, or true Ethmoid (Eth.), which abuts, in front, upon 

 the frontal and nasal bones ; behind, upon the presphenoid ; 

 and below, upon a rod-like mass of cartilage, which becomes 

 connected with the septum narium and the prem axillary bones 

 anteriorly and inferiorly, and is obliterated with age. 



The inferior and posterior part of the septum is constituted 

 by a bone with a gutter-like upper and anterior boundary, 

 which embraces the whole rounded inferior and posterior edge 

 of the cartilage in question, and thus extends from the under- 

 surface of the basi-sphenoid, posteriorly and superiorly, to the 

 middle of the roof of the bony palate, anteriorly and inferiorly. 

 This bone is the Vomer (Vo., Fig. 50). 



Thus there are three bones in the hasi-cranial axis, — the 

 basi-occipital, basi-sphenoid, and presphenoid ; and there are 

 two bones in the hasi-facial axis, — the ethmoid and the vomer ; 

 the essential difference between these two sets of bones being 

 that the former constitute the middle part of the floor of 

 the brain- case, while the latter are altogether excluded there- 

 from. 



^Ye may now turn to the upper arches of the skull, or those 

 bones which form the Avails and roof of the brain-case. In 

 the young skull from which the Figures 52 and 53 are taken, the 

 postero-lateral margins of the basi-occipital are united with 

 the rest of the occipital bone, only by synchondrosis. The 

 parts of the latter which are thus united with the basi-occipital, 

 and which limit the sides of the great occipital foramen, are 

 primitively distinct bones, -the Ex-occipitals (Eo.); while the 



