1. Though the force tends to travel in the direction of the applied force, yet the convexity of the skull allows of the dissemination of that force over a large superficial area.
2. The intervention of cartilage or fibrous tissue between two or more of the component bones of the vault tends to diminish the intensity of the force, to break it up and to alter its direction.
3. The bony ridges, along which the forces tend to travel, themselves terminate blindly (see [Fig. 28]). Thus,
(a) Forces passing from the frontal region converge, more or less, to the crista galli.
(b) Forces from the external angular frontal process pass along the wings of the sphenoid bone to the anterior clinoid process.
(c) Forces from the auditory region are projected along the summit of the petrous bone towards the apex of that process and to the posterior clinoid process.
(d) Forces applied to the occipital region travel inwards along the internal occipital crest to the strengthened margins of the foramen magnum, or are projected outwards along the lateral sinus ridges. In the former case, the force either passes forwards towards the dorsum ephipii and so again reaches the posterior clinoid process, or is directed more laterally towards the jugular process of the occipital bone, there meeting the fibrous tissue intervening between that process and the corresponding part of the temporal bone.
4. All forces, whether transmitted along the internal occipital crest, the temporal bone, the sphenoidal wings, or the crista galli of the ethmoid, are further transmitted to the dura mater attached to those prominences and ridges. The dura mater undoubtedly plays an important part in the reception and transmission of the forces.
5. The forces all show a tendency to converge towards the pituitary region, the great ‘water-cushion’ of the brain—a region bounded by the clinoid processes. That these processes receive a considerable part of the forces transmitted is confirmed by the fact that they are frequently torn away from their basic attachments. This is especially the case with respect to the attenuated base of the anterior clinoid process.
It is obvious, therefore, that forces tend to be transmitted from the vault to the base, and yet the base is, in many respects, the weakest part of the skull. It is perforated by numerous foramina, it is hollowed out in places for the formation of air sinuses and for the reception of the integral portions of the auditory apparatus. Furthermore, it presents a more or less plane surface, one differing in all respects from the marked convexity of the vault. Those forces, therefore, which are received by the base of the skull are not subjected to that diffusion which forms so conspicuous a feature in the case of the vault.