THE STRUCTURE OF THE HUMAN SKULL. 119 



those chambers as compared with the third. The sensory 

 apparatuses of the nose and of the ear are firmly fixed to, or 

 within, the bony chambers in which they are lodged. That of 

 the eye, on the other hand, is freely moveable within the orbit. 

 An axis, upper and lower arches, chambers for the sensory 

 organs, — such are, speaking generally, the components of the 

 skull. The special study of these components may be best 

 commenced from the cranio-facial axis. Viewed either from 

 above (Fig. 52) or from below (Fig. 53), the cranio-facial axis is 

 seen to be depressed, or flattened from above down wards, 

 behind, and thick and nearly quadrate in the middle ; while, 

 in front, it is so much compressed, or flattened from side to 

 side, that it takes the shape of a thin vertical plate. In such a 

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

 the depressed hindermost division of the axis is united with the 

 rest, and with the bones EO, EO, only by synchondroses ; and 

 is readily separable, in the dry skull, as a distinct bone, which 

 is termed the Basi-oecijntal (B 0). This basi-occipital furnishes 

 the front boundary of the occipital foramen ; and its postero- 

 lateral parts, where they abut against the bones EO, contribute, 



\*y 



Fig. 52. — Cranio-facial axis and lateral elements of the superior arches of a human skull 

 viewed from above. — a, the spheno-occipital synchondrosis ; 6, the ethmo-sphenoid 

 synchondrosis ; c, the tuberculum sella, indicating the line of demarcation between 

 the basi-sphenoid and the presphenoid ; d, the linjulce sphcnoidalcs. 



