The facial nerve, therefore, escapes direct injury except in so far that the ganglion may be compressed by blood-clot or fragments of bone. Partial loss of function results. In most cases the blood is absorbed and a complete recovery may be anticipated. The degree of deafness is directly proportionate to the damage incurred by the membrana tympani and ossicles. [Illustration: FIG. 39. TO SHOW THE RELATION OF A TYPICAL BASIC FRACTURE TO THE MIDDLE EAR AND ITS ADJUNCTS. A, Malleus; B, Middle ear and aditus; C, Geniculate ganglion (facial nerve); D, Groove for great superficial petrosal nerve; E, Canal for tensor tympani muscle; F, Processus cochleariformis; G, Eustachian tube; H, External auditory meatus; I, Membrana tympani; J, Mastoid cells.] In the second group of cases a different picture is obtained. Usually the result of blows applied to the occipital region, the fracture traverses the thin cerebellar fossa towards the outer angle of the jugular foramen, thence cutting across the petrous bone, external to the internal auditory meatus, and terminating, usually by comminution, in the tegmen tympani. It is in the transpetrous part of the fracture that the damage is done, for, not only is the facial nerve cut across in the region of the ganglion, but the auditory apparatus is also severed into two parts. The exact line of the fracture is shown in Figs. 40 and 41. [Illustration: A B FIG. 40. TO SHOW THE RELATION OF BASIC FRACTURES TO THE PETROUS BONE. A, The basic fracture, resulting from a force applied to the left occipital region, follows the course depicted in Fig. 36. B, The inner half of the petrous bone, being loose, is thrown forward so as to show the relation of the fracture to the integral parts of the petrous bone.] In this class of fracture, though facial paralysis and deafness are both immediate in onset and permanent in duration, there is, in many cases, no bleeding from the ear as the membrana tympani may be uninjured. The facial nerve may also be implicated in that rare type of basic fracture which was described by Lèon Boullet in 1878, under the title of ‘Fracture of the Mastoid portion of the Temporal bone’. This fracture is fully described on p. 108. It will suffice to mention that the mastoid process may be torn away from the base of the skull, the facial nerve being lacerated as it descends the aqueductus Fallopii. Bilateral facial paralysis is exceedingly rare. Two cases were described by the late Professor von Bergmann. Its occurrence is pathognomonic of the typical basic fracture (see p. 84). [Illustration: A B FIG. 41. TO SHOW THE RELATION OF BASIC FRACTURES TO THE PETROUS BONE. A shows the course pursued by an antero-posterior fracture of the petrous bone. Note that it lies external to the internal auditory meatus. B shows--enlarged--the inner aspect of the outer fragment. Note the relation of the fracture to the semicircular canals, and that the membrana tympani and ossicles are quite uninjured.]