If a vertical and transverse section be taken through the cranium, in such a manner that the plane of the section shall traverse both external auditory meatuses, the skull will be divided into two unequal portions-an anterior, larger, and a posterior, smaller. The former, if viewed from behind, will present the appearance represented in Fig. 48.

Fig. 48.-Anterior half of the skull of a young person (six or seven years of age) transversely bisected. The temporal bone (T) on each side is left in outline, and the contour of the alisphenoid is supposed to be seen through it.-II, optic foramina between the roots of the orbito-sphenoids; V, foramen ovale for the third division of the trigeminal; N indicates the nasal chamber; Mx is placed in the buccal chamber.

A stout median floor (BS) whence lateral continuations (AS) are prolonged to meet an arched roof (Pa), divides a capacious upper chamber, which, during life, lodged a part of the brain, from a lower chamber, formed by the bones of the face. This lower chamber itself is again separable into two

parts, an upper, divided into two by a median septum, the nasal passages; and a lower, the oral cavity.

The posterior portion of the bisected skull (Fig. 49) presents, in like manner, a strong floor (BO) and a large upper chamber for the lodgment of parts of the brain; but the lower chamber seems at first to be absent in the skeleton, being represented, in fact, only by the styloid processes (St), the so-called stylohyoid ligaments, and the hyoidean bone (Hy) which is suspended by these ligaments to the skull.

Fig. 49. The posterior half of the transversely bisected skull, Fig. 48.-B.O., the basioccipital; E.O., E.O., the ex-occipitals; T, the temporal bone left in outline; 0.F., occipital foramen; VII., canal for the portio dura and portio mollis; IX, foramen for the ninth or hypoglossal nerve.

A longitudinal and vertical section of the skull (Fig. 50) enables us to observe the same relations of the parts from another point of view. The central bones (BO, BS, PS, Eth., Vo), which lie between the arches of the brain-case above, and the arches of the face below, are, in such a section, found to

constitute a continuous series, from the occipital foramen to the anterior extremity of the nasal passage, which, as it forms the common centre or axis, not only for the bones of the brain

Fig. 50.-Longitudinal and vertical section of a Human Skull.-* The sella turcica. Au. The position of the superior and posterior vertical semicircular canals. I., II., V., VIII, IX. The exit of the olfactory, optic, third division of the fifth, eighth, and ninth nerves. Vo., the Vomer.

case or cranium proper, but also for those of the face, may be termed the Cranio-facial axis.

'It will be useful to divide this axis into two portions,-a

posterior basi-cranial (BO, BS, PS), which forms the centre of the floor of the proper cranial cavity; and an anterior, basifacial (Eth., Vo.), which constitutes the axis of the front part of

Fig. 51.-Front view of the skull, the halves of which are shown in Figs. 48 and 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, are placed symmetrically upon each side of the double bony box thus described. Of these, two pair are best seen in a front view of the skull (Fig. 51), the inner pair being the olfactory, or nasal chambers (N), the outer pair, the orbits (Or). The other pair are better displayed in the transverse sections, Fig. 48 and Fig. 49, and are formed by the temporal bones of anatomists (T, T), and especially by the petrous and mastoid portions of those bones.

There is an obvious difference between the relations of these sensory chambers to the contained sensory organ, in two of

these 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 downwards, 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-occipital (BO). This basi-occipital furnishes the front boundary of the occipital foramen; and its posterolateral parts, where they abut against the bones EO, contribute,

Fig. 52.-Cranio-facial axis and lateral elements of the superior arches of a human skull viewed from above.-a, the spheno-occipital synchondrosis; b, the ethmo-sphenoid synchondrosis; c, the tuberculum sella, indicating the line of demarcation between the basi-sphenoid and the presphenoid; d, the lingulæ sphenoidales.