The number of centers of ossification in the skull is tolerably constant; each bone having a certain number. (Thus the occipital has 7 centers, the temporal 5, the sphenoid 12, etc.; the total number being about 59). In addition to these, centers frequently occur in the course of the sutures, giving rise to independent pieces, which are culled the oaa tripetra, or the Wormian bolos: They are regarded by Humphry as stop-gaps, developed in the membranous covering of the brain, when the extension of the regular osseous nuclei is likely, for some reason, to lie insufficient to cover in the cranial cavity; and this view is supported by the observation that, in cases of rickets and hydrecephalus, the Wormian bones are especially abundant.
After the sutures have been formed, and the skull has acquired a certain thick ness, a process of resorption commences in the interior of the bones, and reduces tae originally dense structure to a more or less cellular or cancellated state. The interior thus altered is called the diploe, and by this change the weight of the skull is much diminished, while its strength is scarcely affected.
The dipliie usually begins to be apparent about the tenth year, and is most developed in those skulls which are thickest. Dr. Humphry has observed it to be especially thick in idiots, and where the brain is small. " Hence," be observes, "the propriety of the term thick-headed, as a synonym for stupid. derives some confirmation from anatomy." A continuation of the same process of resorption, which causes the diploe, gives rise to the formation of the cavities known as the frontal and sphenoid sinuses. The formation of the diploe divides the walls of the cranium into three lavers, viz., an outer tough layer; an inner dense, brittle, and somewhat glass-like layer, known as the vitreous table or layer; and the intervening cancellous diploe. The vitreous table being more brittle than the outer layer, is apt to be fissured to a greater extent in fracture of the skull; and is even sometimes broken while the outer layer, which received the blow, has remained entire; although the diploe must have great power in lessening the concussions trans mitted from the outer to the inner layer of the skull. The growth of the skull after the seventh year proceeds slowly; but a slight increase goes on to about the age of twenty. The skull-bones are freely supplied with blood from arteries which puss from the dura mater internally and the pericranium externally, through the numerous foramina observed on both surfaces; the blood being returned by veins which take various di rections The'fact that concussion of the brain scarcely ever proves fatal, unless there is also fracture of the skull, affords the most distinct evidence that the skull is constructed in such a manner that so long as it maintains its integrity, it is able to protect its con tents from serious lesion. This marvelous protective power is due to its rounded shape whereby its strength is increased, and in consequence of which blows tend to glide off it, without doing material damage. Moreover, the curved lines or ridges which may be traced round the skull tend to strengthen it. The weakest part of the skull is at the
base. Hence, notwithstanding its removal from exposure to direct injury and the pro tection afforded by the soft parts. fmeture takes place more frequently at the base than at any other part of the skull, fracture often taking place here even when the skull was not broken at the part struck. There are two points in the architecture of the bones of the face which deserve especial notice, viz., (1) the great strength of the nasal arch, and (2) the immobility of the upper jaw, which is fixed by three buttresses, the na sal, the zygomatic, and the pterygoid.
The base of the Skull, whether seen from within or from below, presents many objects of physiological interest in relation to the nervous system. As seen from within, the base presents on each side three fossre, corresponding to the interior and middle lobes of the cerebrum and to the cerebellum. These fossfe are marked, as is the whole skullcap, by the cerebral convolutions, and they contain numerous " foramina" and "fissures" which give passage to various sets of nerves and blood-vessels. The external or outer surface of the base of the skull, if we consider it from before backward is formed by the palate processes of the superior maxillary and palate bones; the vomer; the pterygoid and spinons processes of the sphenoid and part of its body; the under surface of the temporal bones, and by the occipital bone. The most important of the parts which it presents are named in the description of Fig. 3.
The anterior region of the skull, which 'forms the face, is of an irregularly oval form, and the bones are so arranged as to inclose the cavities for the eyes, the nose. and the mouth. and to give strength to the apparatus for masticating the food. The size of the face and the capacity of the cranial cavity stand in an inverse ratio to one another, as may be readily seen by comparing vertical sections (through the menial line) of human and other mammalian skulls; and if, in place of mammalian skulls, we take sk.,1Is of lower vertebrates (the crocodile, for example), this ratio is far more striking. In man the face is at its minimum as compared with the cranial cavity, chiefly in consequence of the facial bones being arranged in a nearly vertical manner beneath the cranium, instead of projecting in front of it. The human face is also remarkable for its relatively great breadth, which allows the orbits for the reception of the eyes to be placed in front instead of on the sides of the head, and renders their inner walls nearly parallel. "This parallelism," says Dr. Ilumphry, "in man is associated with the parallelism of the optic axes, and contributes to that clear, accurate, and steady vision which results from the ready convergence of the eyes upon every object." Each orbit is of a pyramidal ream, with the apex behind, and is composed of seven bones—viz , the frontal, ethmoid, lach rymal. sphenoid, superior maxillary-, malar, and palate, which last contributes ver7 slightly to the human orbit, but Is an important constituent in the orbit of many ani mals. The nasal cavities have been sufficiently described in the article N 05E.