[Illustration: FIG. 30. PLAN OF THE BASE OF THE SKULL. _a_, _a_, The transverse pre-condyloid line; _a′_, _a′_, The line pursued, in whole or in part, by the ‘typical’ basic fracture.] Any blow delivered at or near the basic level tends _primarily to involve the weaker area, the base, passing secondarily upwards on to the vault_. One may go even so far as to say that in most combined vault and basic fractures, the vault fracture is a secondary development, the basic fracture being the primary lesion. There is, however, still another important anatomical feature bearing on the mechanism of basic fractures, one that must necessarily come into force in the greater number of such fractures. The base of the skull may be said to consist of two parts, one lying anterior to the condyles of the occipital bone, the other posterior to and including the condyles with their vertebral attachment. These two segments are united to one another by a weak chain--represented by a line drawn from one external auditory meatus to the other, with, as a connecting link, the sphenoidal sinus in the middle line. When the base of the skull is viewed from below, it will be seen that the weak line includes both Glaserian fissures, both petro-sphenoidal sutures, both foramina lacera media, with the sphenoidal sinus again as a connecting link. The two parts of the skull are, to all intents and purposes, merely cemented together by the union of the basi-sphenoid and basi-occiput. Consequently, if a blow be received on the antero-lateral region of the head, the anterior segment tends to be split off from the more fixed posterior part, the fracture following the weak line previously indicated. This weak line occupies so important a position in the mechanism of basic fractures that careful observation will show that the greater number of middle fossa fractures follow that line, wholly or in part. Such a fracture of the middle fossa may be termed ‘the typical fracture of the base of the skull’ (see Figs. 30, 34 and 39). =Summary of theories.= Aran’s theory of irradiation, with certain modifications, accounts satisfactorily for about 30 per cent. of basic fractures. The contre-coup theory may be rejected entirely. The bursting and compression theories are unsatisfactory, accounting for not more than about 5 per cent. of fractures. The majority of cases result from direct violence applied at or near the basic level, the fracture passing across the base in the general direction of the applied force, but not necessarily in the same straight line. Up to this point certain facts and theories have been discussed, such as bear on the general mechanism of basic fractures. It now remains to consider other factors that exercise influence on the general direction of the fracture.