The cerebral nerves pass out of the embryonic skull between the various parts of the chondro-cranium, and the foramina of exit in the adult skull may be divided into groups on this basis. Thus we get foramina (a) between various distinct parts of the skull, as the jugular between petrous and occipital ; (6) between different parts of what is one bone (in the adult), as the anterior condyloid foramen between basi- and ex- occipital. A class occurs as a subdivision of [a), the foramen being formed by ossification extending from one of the bordering elements round the issuing structure : thus we may get the foramen ovale, for instance, as a secondary enclosure of a nerve originally passing between alisphenoid and periotic capsule, or in some cases, as in the canalis innominatus of the sphenoid, the enclosure may only take place occasionally.
We have seen that the skull can be broadly divided into a dorsal and lateral group of bones developed in relation with the capsule of the brain, and a ventral skeleton of a visceral origin.
So far the division, given above, is founded simply on the embryonic development and is therefore unassailable. But when we come to inquire into the remoter values of the parts concerned, and to compare the human skull with those of lower types, the situation becomes more complicated and bristles with difficulties.
So far as the mammals are concerned, the comparison is easy and straightforward, but below this class the difficulties begin. In all classes of vertebrate skulls the uncertainties are not so much with regard to the bones of the brain capsule, nor with reference to the greater part of the visceral skeleton but are mainly associated with the comparative values of the prechordal and nasal capsular regions-in fact, they may in general be said to centre round the nose and roof of the mouth.
It would be out of place in the present work to enter on a consideration of the questions involved, and it will be sufficient to mention that the membrane bones are generally thought to represent dermal structures which, originally placed superficial to the coverings of the brain, as this expanded in evolution came to occupy a deeper situation to fill the call for increased covering : thus in the Elasmobranch fishes the brain case is a continuous cartilaginous one, but as we proceed further up the vertebrate scale we find the cartilage becoming confined to a relatively lessening basal area, while its place on the vertex is taken by membrane bones presumably derived from more superficial tissues. A similar view is taken of the formation of the jaws, but here the question is complicated by the failure of the human bones to show centres of ossification corresponding in number with the presumed dermal bones concerned : thus each half of the lower jaw, for example, has only one centre instead of a number in agreement with the number of elements (angular, splenial, etc.) that are supposed to have sunk into the deeper tissues of the mandibular arch to make the bone. It seems doubtful, moreover, whether certain cartilage bones have not been replaced by membrane bones which from their positions could not have come from the surface.
The chondro-cranium is replaced by the osseous cranium. The times of appearance of the oss'fic centres vary in d'fferent bones ; but two main periods of activity in this direction are to be noticed, the first about the end of the second month, or beginning of the third month and the second about the fifth month. All the bones formed in the cartilaginous base, excepting the special sense capsules, begin to ossify in the first period, as do also all the membrane bones of the vault ; the lingula begins a little later, in the fourth month. The bones formed in the special sense capsules, 6n the other hand, belong to the second period, including the ethmoid, inferior turbinal, sphenoidal turbinal, and petrous centres : the centre for the vertical plate of the ethmoid is even later, occurring after birth. The lachrymal, nasal and vomer, membrane bones lying against the nasal capsule, belong to the first period, as also does the tympanic bone, a membrane bone applied to the periotic capsule in part. In the visceral skeleton there is greater variation, the mandible showing centres in the fifth week, before the centres of the maxillary process, the derivatives of which belong to the first period ; this includes the maxilla, palate, internal pterygoid plate, and malar.* But the visceral bones further back are much later than the second period, the tympano-hyal not showing a centre until a month or so before birth and the stylo-hyal not until some months after that event : the upper ends of these bars, however, come into the second period by commencing to ossify as the small bones of the middle ear in the fifth month.
Summing up this account of the ossification, it may be said that all the membrane bones begin to ossify in the first period, the cartilaginous cranial bones partly in the first and partly in the second (special sense capsules), and the cartilaginous visceral bones considerably later.
So it comes about that, save in the perpendicular plate of ethmoid and the styloid process, all the bones of the skull are in process of formation for some time before birth, although the various elements concerned have not joined with each other in all cases-to make the bones as we see them in the older skull. Thus the bony post-, ex-, and basi- occiput are distinct ; the basiocciput is separated by cartilage from the basisphenoid, this partially from the presphenoid, the squamous from the petrous temporal, and the two halves of the lower jaw are not joined in the middle line. In addition the membrane bones of the cranial vault, though they grow rapidly, do not meet each other over the quickly-growing brain, and at birth are separated by lines and areas of fibrous tissue where the periosteal covering of the bones becomes directly continuous with the dura mater lying immediately underneath, j The largest of these interosseous fibrous areas occur at the angles of the parietal bone : the areas are termed fontanelles, and these larger ones are the anterior and posterior median at the bregma and lambda respectively, and the anterior and posterior lateral at the plerion and asterion (Fig. 212). The bregmatic fontanelle is four-sided, between the two parietals and two halves of the frontal ; the lambdoid is triangular, between occipital and parietals ; and the two lateral intervals are irregular in shape, that at the asterion sometimes extending back for a little distance between the post- and supra- occipital.