It is by the bases of these doines thus thrusting against a solid frame-work, that the cranium is endowed with the power of re sisting lateral shocks whether they approach from before or behind ; and it is not, as some allege, simply by the mobility of the head, that it withstands blows, which, if it were fixed, would fracture it.
There yet remains to be noticed an impor tant part of this skeleton or frame-work ; that which bears upon the spine, and resists the force transmitted through it. At the bottom of the pit containing the cerebellum, there is an elliptical opening (the foramen magnum), the margin of which is very dense; this opening is provided underneath with two tubercles (the articulating processes), by which it rests on the vertebral column; from these tubercles a curved rib on each side (the lateral process of the oc cipital bone and the mastoid of the temporal) extends upwards and outwards to the extremity of the posterior lateral rib ; the segment of the margin of the opening which is anterior to the tubercles, is prolonged upwards and forwards, in the form of a broad pillar (the basilar pro cess), to the back part of the common centre; the segment which is behind the tubercles sends off, at its back part, a spine (the inferior limb of the internal crucial spine), which ends at the centre of the horizontal arch, at the point where the superior longitudinal rib terminates ; and this point of confluence of the forces from below, from above, and from behind, is strength ened by a nodule (the internal occipital protu berance). The frame-work of the cerebellar cavity is thus connected with that of the general cavity; anteriorly, to the body of the sphenoid bone; posteriorly, to the tubercle of the occi pital; and, laterally, to the extremities of the petrous processes of the temporal bones. In both of them it will be seen that they occupy spaces between the grand divisions of the ner vous matter, which latter is, therefore, removed from .the chance of sustaining injury by shocks,
much more completely than it could have been had the parietes been submitted to a progres sive augmentation of substance from above downwards. As it is, the spaces in which the nervous matter reposes are thin and frequently diaphanous ; and, were they situated in un protected parts, would be perforated by the slightest force.
Dining a considerable period of life the S113- ject enjoys additional protection from the slight yielding of the bones, and from the cartilage which intervenes especially at the base. Pres sure applied on the vertex would tend to disjoin the parietal bones from each other, and from the frontal and occipital bones. This .the pe culiar nature of the articulations forbids, and the longitudinal rib chiefly, and the expanded portion of the bones themselves in part, convey the force downwards, the former forwards through the median line of the ethmoid to the front of the sphenoid, and backwards through the superior and inferior limbs of the crucial spine of the occiput, traversing the foramen magnum, and passing through the basilar process to the back of the sphenoid bone : the latter fbrwards through the frontal bone to the small and great wings, and, through them, to the body of the sphenoid ; and backwards through the parietal and occipital to the lateral limbs of the crucial spine. The parietals convey it down the sides to the great wing of the sphenoid and the mas toid process of the temporal bone, from which it is transmitted to the common centre ; and the slight rotation which is permitted to the temporal bone, (and which has already been alluded to,) materially tends to break the force in its transit. Nor is there any imperfection in this apparent inclination of the parietals to an outward divergence, for the squamous process of the temporal bone which overlaps each be tween its two fixed points is strongly supported on its outer side by the temporal muscle.