[Illustration: FIG. 28. DIAGRAM ILLUSTRATING THE LINES ALONG WHICH FORCES RECEIVED ON THE VAULT ARE TRANSMITTED TO THE BASE. (For further description, see text.)] Further, not only does the skull vary in density in its several parts, but it is also ribbed and strengthened by various bony bars and buttresses that pass up from base to vault (see Fig. 28). These ‘ribbings’ are seen to extend upwards from the crista galli, from the external angular frontal process, from the auditory region, and from the occipital protuberance. Presumably, these ‘ribbings’ were so constituted for a definite purpose; in any case, it is clear that they play an important part in the reception and conduction of forces to the base of the skull. It is apparent, moreover, that the parts intervening between these ‘ribbings’ are liable to injury in direct proportion to their general position and strength. The deep groovings of the bone for the reception of the middle meningeal artery afford an additional source of weakness to the bone in the squamo-temporal region. (See Fig. 50). [Illustration: FIG. 29 A. THE BASE OF THE SKULL.] [Illustration: FIG. 29 B. THE BASE OF THE SKULL AS SEEN ON TRANSILLUMINATION.] Further reference will be made to the relative strengths of the various regions of the skull. Sufficient has been said to show that nature has provided the skull with various paths by means of which forces applied to the vault can be conducted and distributed to the base. Before, however, proceeding to discuss the effects produced on the base of the skull, it is necessary to add that nature provides other methods by means of which the intensity of a blow, delivered over the vertex of the skull, is diminished. The forces are broken up and distributed in the following manner:-- 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.