In such cases the fracture may extend on each side up on to the vault in such a manner that the two segments are merely united by the soft parts; whether the fracture be so complete or not, a more minute examination of the line of separation will evidence many interesting points. An inspection of the anterior aspect of the posterior fragment shows that the fracture passes just in front of the geniculate ganglion of the facial nerve, the ganglion being laid bare, whilst its petrosal branches are usually torn. The fracture also passes anterior to the Eustachian tube and the horizontal part of the internal carotid artery. On examining this posterior fragment the following structures will be seen, passing from without inwards: the posterior half of the external auditory meatus, the mastoid antrum, the lacerated membrane and the ossicles of the middle ear, the geniculate ganglion of the facial nerve, the Eustachian tube, the horizontal part of the internal carotid artery, the Gasserian ganglion, and the posterior half of the sphenoidal sinus in the middle line (see also Fig. 39).
After reaching the sphenoidal body, the alternative course for the fracture to pursue is to pass towards the opposite sphenoidal fissure and, tearing off the anterior clinoid process, to be directed across the anterior fossa, parallel to the original direction but not in the same straight line.
[Illustration: FIG. 34. DIAGRAM OF LINES PURSUED BY BASIC FRACTURES. Force applied to the region of the external ear. _a_ ... _a_, The ‘typical’ basic fracture (see also Fig. 30).]
4. Force applied to The fracture follows the occipito-mastoid suture the mastoid region. to the jugular foramen, and is again continued on the opposite side of that foramen along the petro-occipital suture towards the apex of the petrous bone. It then passes across the sphenoidal body to the sphenoidal fissure of the opposite side, and so across the anterior fossa. It is especially common in this particular variety of fracture to find fissures diverging from the region of the sphenoidal sinus forwards towards the cribriform plate of the ethmoid, these fissures usually passing between the optic foramina.
This fracture is also peculiar in so much that, when the degree of separation along the occipito-mastoid suture is excessive, there is special liability to a tearing of the lateral sinus wall as the sinus begins to turn downwards and inwards.
