Skull

From the internal occipital protuberance the two wide grooves for the lateral venous sinuses run nearly horizontally outward till they reach the posterior inferior angles of the parietal bones; here they turn downward with an S-shaped curve, grooving the mastoid portion of the temporal and later on the exoccipital bones, until they reach the jugular foramina. To the edges of the horizontal parts of these grooves, and to the upper edge of the petrous bones the tentorium cerebelli is attached.

The Skull in Sagittal Section.

If the skull be sawn down just to the right of the mid line and the left half be looked at, the appearance will be that reproduced in fig. 5. The section of the cranial bones shows that they are formed of an outer and inner table of hard bone, while between the two is a layer of can cellous tissue called the diploe. In certain places the diploe is in vaded by ingrowths from the air passage which separate the two tables and form the air sinuses of the skull, though it is important not to confuse these with the intracranial blood or venous sinuses. In the section under consideration two of these spaces, the frontal and the sphenoidal air sinuses are seen. Behind the frontal sinus is the crista galli already mentioned, while below is the bony sep tum of the nose formed, by the mes-ethmoid plate, the vomer and the line of junction of the palatine processes of the two maxillae and two palate bones. The re-entering angle between the mes ethmoid and vomer is filled in the recent state by the septal cartilage.

Below the face is the inner surface of the body and ramus of the mandible, and half-way down the latter is the inferior dental foramen where the inferior dental branch of the fifth nerve ac companied by its artery passes into the inferior dental canal in the substance of the bone to supply the lower teeth.

If the cut surface of the right half of the skull be looked at, the outer wall of the nasal cavity will be seen with the three turbi nated bones each overhanging its own meatus, but the anatomy of this part has already been dealt with in the article on the olfactory system (q.v.).

For further details see any standard anatomical textbook Quain, Gray, Cunningham, etc.

The notochord (see SKELETON : Spine) extends forward to the ventral surface of the middle cerebral vesicle (see BRAIN) or as far as the place where the dorsum sellae will be. It is partly sur rounded by the mesenchyme just as it is completely in the rest of the axial skeleton, and this mesenchyme extends dorsally on each side to wrap round the nerve cord, which is here the brain. In this way the brain becomes enclosed in a primitive membra nous cranium, the inner part of which persists in its primitive con dition as the dura mater, while the outer part may chondrify, chondrify and ossify, or ossify without a cartilage stage. On each

side of the notochord a basicranial plate of cartilage is formed which soon meets its fellow of the opposite side, and forms the floor of the skull as far forward as the dorsum sellae, and as far back as the external occipital protuberance. Laterally it comes in contact with the mesenchyme surrounding the internal ear, which is also chondrifying to form the cartilaginous periotic cap sule, and the two structures fuse together to form a continuous floor for the back of the skull. In the hinder occipital region of the calf there are evidences of four vertebrae having been incorpo rated with the basicranial plate, that is to say that the plate and its coalesced vertebrae represent five mesodermic somites. The same is true for man. Moreover, the primitive membranous skull shows signs of metameric segmentation in the way in which the cranial nerves pierce the dura mater one behind the other. These segments, however, had lost their distinctness even before the car tilaginous cranium had become developed, so that there is no real segmental value in the elements of this, still less in those of the bony skull. The only place in which segmental elements can be distinguished is in the occipital region, which is in structure transi tional between the head and vertebral column. The notochord, it has been shown, ends just behind the place where the stomodaeum pouches up through the cranial base to form the anterior part of the pituitary body (see BRAIN). Where it ends two curved bars of cartilage are formed, which run forward till they meet the olfac tory capsules, which are also now chondrifying. These bars are the prechordal cartilages or trabeculae cranii and enclose between them the cranio-pharyngeal canal by which the pituitary body ascends, but later on, as they grow, they join together and cut off the pitui tary body from the pharynx. By the growth outward they form the floor of the prechordal part of the chondro-cranium, so that from them is developed that part of the cartilaginous skull which will later on be part of the basisphenoid, the presphenoid, orbito sphenoid and alisphenoid regions. It was assumed that this process held good for man, but later research showed that the anterior part of the base of the skull chondrifies in the same way that ice appears on a pond and that the trabeculae are at no time definite structures. Chondrification of the nasal capsules is later than that of the parts of the skull behind, so that there is a steady progress in the process from the occipital to the ethmoidal region. There is a median centre of chondrification, the mesethmoid cartilage, which projects down into the fronto-nasal process (see OLFAC TORY SYSTEM), and two lateral ectethmoid cartilages which even tually join with tile mesethmoid to form the cartilaginous ethmoid.