pressions, is the lower part of the fossa for the lateral sinus on each side.The inner surface of the cuneiform apophyse is made concave for the reception of the medulla oblongata, and of the basilar artery.—A furrow is made on each side, near the edges of this process, by a sinus of the dura mater, which empties itself into the lateral sinus.
The holes of this bone are commonly five proper, and two common to it and to the temporal bones.—The first of the proper holes, called foramen magnum, from its size, is immediately behind the wedge-like process, and allows a passage to the medulla oblongata, nervi accessorii, to the vertebral arteries, and sometimes to the vertebral veins.At each side of this great hole, near its fore-part, and immediately above the condyles, we always find a hole, sometimes two, which soon unite again into one that opens externally; through these the ninth pair of nerves go out of the skull.—The fourth and fifth holes pierce from behind the condyle of each side, into the fossæ of the lateral sinuses; they serve for the passage of the cervical veins to these sinuses. Often one of these holes is wanting, sometimes both, when the veins pass through the great foramen.Besides these five, we frequently meet with other holes near the edges of this bone, for the transmission of veins; but their number and diameter are very uncertain. The two common foramina are the large irregular holes, one in each side, between the sides of the cuneiform process, and the edges of the petrous bones. In a recent subject, a strong membrane runs cross from one side to the other of each of these holes.
The occipital bone is among the thickest of the cranium, though unequally so; for it is stronger above, where it has no other defence than the common teguments, than it is below, where, being pressed by the lobes of the brain and cerebellum on one side, and by the action of the muscles on the other, it is so very thin, as to be diaphanous in many skulls.
The occipital bone is joined above to the ossa parietalia and triquetra when present, by the lambdoid suture;—laterally to the temporal bones, by the additamenta of the lambdoid suture;—below to the sphenoid bone, by the end of its cuneiform process, in the same way that epiphyses and their bones are joined.The os occipitis is joined by a double articulation to the first vertebra of the neck, each condyle being received into a superior oblique process of that vertebra.
Os Ethmoides, or the sieve-like bone, has got its name from the great number of small holes with which that part of it first taken notice of is pierced. When this bone is entire, the figure of it is not easily described; but, by a detail of its several parts, some idea may be afforded of the whole; and therefore we shall distinguish it into the cribriform lamella with its process, the nasal lamella, cellulæ, and ossa spongiosa.
The thin horizontal lamella, is all (except its back part) pierced obliquely by a great number of small holes, through which the filaments of the olfactory nerves pass.—From the middle of the internal side of this plate, a thick process rises upwards, and, being highest at the fore-part, gradually becomes lower, as it is extended backwards. From some resemblance which this process was imagined to have to a cock's comb, it has been called crista galli. The falx is connected to its ridge, and to the unperforated part of the cribriform plate.—When the crista is broke, its base is sometimes found to be hollow, with its cavity opening into the nose.
From the middle of the outer surface of the cribriform lamella, a thin solid plate is extended downwards and forwards, having the same common base with the crista galli. Generally it is not exactly perpendicular, but is inclined to one side or other, and therefore divides the cavity of the nose unequally. Its inclination to one side, and flexure in the middle, is sometimes so great, that it fills up a large share of one of the nostrils, and has been mistook for a polypus there.—It is thin at its rise, and rather still thinner in its middle; yet afterwards, towards its lower edge, it becomes thicker, that its conjunction with the bones and middle cartilage of the nose might be firmer.
At a little distance from each side of this external process, a cellular and spongy bony substance depends from the cribriform plate. The number and figure of the cells in this irregular process of each side, are very uncertain; only the cells open into each other, and into the cavity of the nose: The uppermost, which are below the aperture of the frontal sinuses, are formed like funnels.—The outer surface of those cells is smooth and plain, where this bone assists in composing the orbit; at which place, on each side, it has got the name of os planum; on the upper edge of which, a small notch or two may sometimes be observed, which go to the formation of the internal orbitar holes.
Below the cells of each side, a thin plate is extended inwards, and then bending down, it becomes thick, and of a spongy texture.—This spongy part is triangular, with a streight upper edge placed horizontally, an anterior one slanting from above, downwards and forwards, and with a pendulous convex one below.—The upper and lower edges terminate in a sharp point behind.—The side of this pendulous spongy part next to the septum narium is convex, and its external side is concave.—These two processes of the ethmoid bone have got the name of ossa spongiosa, or turbinata superiora, from their substance, figure, and situation.
All the prominencies, cavities and meanders of this ethmoid bone, are covered with a continuation of the membrane of the nostrils, in a recent subject.—Its horizontal cribriform plate is lodged between the orbitar processes of the frontal bone, to which it is joined by the ethmoid suture, except at the back-part, where it is connected with the cuneiform bone, by a suture common to both these bones.—Where the ossa plana are contiguous to the frontal bone within the orbit, their conjunction is reckoned part of the transverse suture.—Farther forward than the ossa plana, the cells are covered by the ossa unguis, which are not only contiguous to these cells, but cannot be separated from them, without breaking the bony substance.—Below the ossa unguis and plana, these cells and ossa spongiosa are overlopped by the maxillary bones.—The cellular part of each palate-bone is contiguous to each os planum and cells backwards.—The
