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Show 118 ON rrHE VERTEBRA'rE SKULL. po~terior basi-eranial (BO, BS, PS), which form8 tho eentre of the floor of' the proper cranial cavity ; and an anterior, basifac, ial (Eth., Vo.), which constitutes tho n~ iH f th ' frout part of the face. Fig. 51. - -·---- ---- Fig. 51.-Frout view of the skull, the halve· of which arc .hown in Fig:;. 48 anJ 49.N, nasal chamber; Or, orbit. The nasal bones are removed, and so much of' the upper and lower jaws as is necessary to show the permanent teeth. Three pairs of chambers, destined for the lodgment of the organs of the higher senses, arc pla eel yn1metrically upon each side of the double Lony box thus cl scribed. Of these, two pair are best seen in a front view of the kull (Fig. 51), the inner pair being the olfactory, or nasal ehambers (N), the outer pair, the orbits (Or). The other pair aro bettor displayed in the transverse sections, _Fig. 48 and Fig. 49, and are formed by the temporal bones of anatomists (T, Tl), and especially by the p'etrous and mastoid portions of tho ·e bones. There is an obvious difference Let ween the relations of these sensory chambers to the contained ·ensory organ, in two of 'l'HE STRUCTURE OF 'l'HE HUMAN SKULL. 119 these chambers as compared with the third. 11he s nsory apparatuses of the nose and of the ear are firmly fixed to_, or within, the bony chambers in whi ·h they are lodged. That of the eye, on the other hand, is freely moveable within the orbit. An axis, upper and lower arches, chamb rs for the sensory organs,-such are, ·peaking generally, the components of the skull. The special study of these components may be best commenced fi·om 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; whilt:, in front, it is so much con1pressed, 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 i ·united with the rest, and with the bonos EO, EO, only by ynchondro es; and is readily separable, in the dry skull, as a distinct bone, which is tenned the Basi-oeeipital (B 0). Tllis basi-occipital furnish D the front boundary of the occipital foramen; and its posterolateral parts, where they aLut against the bones EO, contribute, Fig. 52. Fig . .J<:l.-Cranio-facial axis and lateral elements of the superior arch s of a human skull viewed from abovc.-a, the spheno-occipital synchoudrosis; b, the ethmo-splwnoicl synchondrosis ; c, the tubercul-um setlce, indicating the line of <kmarcation between the basi-sphenoid nml the presphenoid ; d, the lin:;ttlQ} splwnoiclulcs. |
