Assuming that the skull involved the pro perties of an arch, its defensive power has by some been attributed to the circumstance of its being of that figure. An arched form, how ever, would serve it only in the case of force descending from above; it would not provide resistance to those severe shocks which are communicated from below, as in jumping, nor protect it from blows that might arrive on its sides.
But the cranium is not an arch, for there are neither piers on which the extremities of that arch could rest, nor abutments to resist their lateral thrust. Supposing a barrel to be sawed lengthwise, and the edges to be connected by a base, if the centre be applied on a column, (the proportion of which to the base is the same as that of the spine to the width of the skull,) it is manifest that, since the extremities of the arch are received on the ends of two long levers which have a common fulcrum, an inconsiderable force would have a tendency to sever them at their junction. On the other hand, if the barrel vvere entire, force would be transmitted through the parietes to a point exactly opposite to that on which it impinged, if it were not dissipated in its transit. Such a degree of force however might be applied, that its vibrations, distributed at the moment of its application, might pass through the entire walls, and, accumulating at one spot, by their intensity cause the fracture of the part. The natural mode of providing against this occur rence would be to strengthen the part in which (from the situation of the organ) these vibra tions might, in general, be expected to concur; and this is the contrivance adopted in the cra nium, for in the centre of its base there is a qua drilateral portion (the body of the sphenoid bone) of characteristic massiveness and strength.
It does not however augment uniformly in its substance from above downwards. The matter is accumulated in dense lines or ribs, which pass to a common centre, and constitute thereby a peculiar skeleton or frame-work of surpassing strength, which admits of the intro duction of a lighter and more fragile structure in the intervening spaces, and resists the shocks that arrive through the spine, from behind or from above.
This frame-work is situated almost entirely in the base; the only part which is in the calvarium being a longitudinal curved line, formed by the ethmoidal process of the sphe noid bone, the crista galli of the ethmoid, the spine of the frontal, the thickened commutual margins of the parietals, and the superior limb of the internal occipital spine. Independently
of this curved rib, the calvarium consists of four ovoidal domes, two on each side; formed, the anterior by the corresponding half of the frontal bone, and the posterior by the parietal. The summits of these domes are their centres of ossification, and their bases abut, partly on the longitudinal rib, and partly on the frame work in the base.
The part to which all the forces tend is the body of the sphenoid bone. From its posterior corners there pass backwards two ribs, (the petrous processes of the temporal bones,) which terminate on the extremities of an arch, (the lateral limbs of the internal crucial spine of the occiput,) which is placed horizontally, and the convexity of which is turned back wards.
This arch and the two ribs which connect it to the centre are in the line in which the oc ciput would strike the ground in falling back wards ; and they further form the brim of the pit which contains the cerebellum, so that the vibrations of force pass in the interstice between that organ and the cerebruni.
From each side of the body of the sphenoid bone there stretches forwards, outwards, and upwards towards the temples, a curved rib, (the anterior pait of the great wing,) and, from the anterior part of the body, a transverse rib which overlays the former. These and the posterior lateral ribs, all of which depart from a common centre, constitute the frame-work of the base which sustains the ovoidal domes of the calvaria. The frontal dome is placed with its summit (the frontal depression) looking backwards, downwards, and inwards ; its mar gin is received, inferiorly on the whole length of the anterior transverse, and on the extremity of the anterior lateral curved rib; towards the middle line, on so much of the longitudinal rib as extends to the parietal bones ; and supe riorly, it is applied against a portion of the base of the parietal dome. It is against these parts that it thrusts, whenever it receive3 .a shock on its summit. The parietal dome is placed with its summit (the parietal depression) looking. downwards and inwards. Below, it is received on the extremities of the lateral ribs ; above, it thrusts against the remainder of the longitudinal rib ; behind, it falls on the corresponding portion of the horizontal arch ; and, in front, it antagonizes the frontal.