All these points tend to show that the base of the skull is more or less unsuited for the reception of severe blows, direct or transmitted, whilst, on the other hand, nature has taken into consideration and provided fairly adequately against the dangers incident to vault injuries.
It is not proposed at this stage to discuss further the relative strength of vault and base. Points, other than those already enumerated, will be brought forward in subsequent sections.
FRACTURES OF THE BASE OF THE SKULL
Fractures of the base of the skull are produced in two ways:—
1. By violence applied directly to the base—perforating wounds of the orbit, bullet-wounds through the mouth and base of skull, the driving inwards of the bones of the face, the driving upwards of the condyle of the jaw (the ‘knock-out’ blow of the pugilist), and, as the result of heavy falls on to the feet or buttocks, the upward driving of the condyles of the occipital bone.
2. By violence applied indirectly—blows applied directly to the vault and transmitted to the base. This variety will be considered first as it receives the greatest prominence in surgical textbooks. Various explanatory theories have been advanced, of which the following are the more important:—
(a) Aran’s theory of irradiation.
This theory states that ‘fractures of the base result as extensions from fractures of the vault, the fracture following the shortest anatomical route to the base’. Although this theory must be accepted as offering a satisfactory explanation for the occurrence of a certain proportion of basic fractures, such, for instance, as result from a blow applied directly to the vertex, it certainly cannot be accepted as accounting for the great majority of basic fractures. The theory was advanced on the hypothesis that basic and vault fractures were necessarily co-existent. That combined lesions of this nature are frequently in evidence is not to be doubted for one instant. It is, however, ‘putting the cart before the horse’ to say that the vault fracture is always the primary lesion. Such is by no means the case.
(b) The bursting and compression theories.
The skull is here regarded as an highly elastic sphere, compression of which leads to diminution in the diameter along the axis of greatest pressure, bulging occurring in other diameters. The bulging exceeding the limits of elasticity a fracture occurs, the line of fracture varying according to the different features present. Thus, when the lines of fracture run parallel to the direction of the compressing force the bone bursts open along the convexity (bursting fractures), and when the lines of fracture run at right angles to the direction of the compressing force a fracture by compression is said to result (compression fractures).