1170 SURFACE AND SURGICAL ANATOMY. 
_ The facial nerve, after entering the aqueduct of Fallopius at the bottom of the 
internal auditory meatus, lies immediately above and behind the foramen ovale, 
between it and the prominence of the external semicircular canal; thence it 
descends almost vertically in the posterior wall of the tympanum ¢ in. behind and 
internal to the lower half of the deepest part of the posterior wall of the external 
osseous canal, and emerges through the stylo-mastoid foramen (Fig. 515). 
In the infant, in consequence of the absence of the mastoid process, the exit of 
the facial nerve from the stylo-nastoid foramen is unprotected and exposed upon 
the lateral rather than upon the basal surface of the skull at a poimt immediately 
behind the posterior segment of the tympanic horse-shoe. It follows, therefore, 
that, in infaney, the incision to expose the antrum should not be curved too far 
downwards and forwards, otherwise the facial nerve may be divided. In the 
infant the position of the mastoid antrum is relatively higher than in the adult, 
Septum of frontal sinuses Crista Galli Left frontal sinus 
Right frontal sinus | Floor of anterior fossa of skull 
\ Anterior part of roof of orbit 
Nasal process of 
superior maxilia 
Anterior extremity of middle _ ee : 
turbinal— ee 
Cartilage of nasal septum- 
Anterior extremity of inferior — 
turbinal ; 
—— Ala nasi 
\ 
Fic. 797.—-VERTICAL CORONAL SECTION THROUGH THE NOSE AND FRONTAL SINUSES. 
because in the former the upper wall of the osseous canal inclines towards the 
vertical plane instead of bemg horizontal. 
To open the mastoid antrum the surgeon makes a curved incision a little behind the 
attachment of the auricle, and chisels or drills away the bone immediately above and 
behind the postero-superior quadrant of the external osseous meatus. In this operation 
the middle fossa of the skull is avoided by keeping below the supra-mastoid crest, the 
lateral sinus by keeping close to the external auditory canal and by chiselling obliquely to 
the surface in opening the mastoid cells, the descending portion of the facial nerve is 
avoided by not encroaching upon the lower half of the deepest part of the posterior wall 
of the osseous canal. In extending the operation from the mastoid antrum through the 
aditus into the tympanic attic, the external semicircular canal and the curve of the faczal 
nerve, Which lie in relation to the inner wall of the aditus, are liable to injury, and must 
be protected either by a curved probe, or better by Stacke’s protector, passed from the 
antrum through the aditus into the tympanic cavity. 
The frontal air sinuses are two cavities situated immediately above the root of 
the nose between the two tables of the frontal bone. Each sinus at its most 
dependent part communicates, by means of the naso-frontal duct, either directly 
with the middle meatus of the nose, or indirectly with that channel through its 
