THE CKANIUM. 



1371 



i nferior half of the deepest part of the posterior wall of the external osseous canal 

 und emerges through the stylo-mastoid foramen (Fig. 10*73). 



In the infant, in consequence of the absence of the mastoid process, the exit of 



,he facial nerve from the stylo-mastoid foramen is unprotected and exposed upon 

 *}he lateral rather than upon the basal surface of 

 j ;he skull, at a point immediately behind the 

 posterior segment of the tympanic horse-shoe. 



tt follows, therefore, that, in infancy, the incision 



;o expose the antrum should not be curved too 



:ar downwards and forwards, otherwise the facial 

 ; lerve may be divided. In the infant the position 



)f the tympanic antrum is relatively higher than 

 J.n the adult, because in the former the upper 

 {.vail of the osseous canal inclines towards the 



vertical plane instead of being horizontal. 



The lymph vessels from the auricle and ex- 

 ternal meatus open into the posterior and anterior 



luricular lymph glands, the latter receiving also 



:he lymph from the middle ear. The efferent 



vessels from those glands open into the superior 



mb-sternomastoid glands ; hence it is that those 



groups of glands are so frequently found to be 



liseased secondary to tuberculosis of the middle 



jar ; and care must be taken not to mistake an 



ibscess in one of the mastoid glands for sub- FIG. 1077. RIGHT FRONTAL SINUS OF 



)eriosteal mastoid suppuration associated with VEBY LARGE DIMENSIONS ; LEFT SINUS 



niddle-ear disease. 



UNOPENED (Logan Turner). 



To open the tympanic 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 

 >ehind the postero-superior quadrant of the external osseous meatus. In this operation 

 he middle fossa of the skull is avoided by keeping below the supra-mastoid crest ; the 

 ransverse sinus, by keeping close to the external acoustic canal and by chiselling obliquely 

 o the surface in opening the mastoid cells ; the descending portion of the facial nerve is 

 ivoided by not encroaching upon the inferior half of the deepest part of the posterior wall 

 >f the osseous canal. In extending the operation from the tympanic antrum through the 

 xlitus into the epitympanic recess, the lateral semicircular canal and the curve of the 

 'acial nerve, which lie in relation to the medial wall of the aditus, are liable to injury, 

 aid must be protected either by a curved probe, or, better, by a Stacke's protector, passed 

 rom the antrum through the aditus into the tympanic cavity. 



The frontal air sinuses are two cavities situated immediately above the root of 

 <he nose between the two tables of the frontal bone. Each sinus at its most 

 lependent part communicates, by means of the naso-frontal duct, either directly 

 vith the middle meatus of the nose, or indirectly with that channel through its 

 nfundibulum. A bony septum, rarely incomplete, separates the two sinuses ; it 

 s usually median in position below, but it may deviate to one or other side above 

 Figs. 1075 and 1076). 



The sinuses vary considerably in their size and shape, independently of the 

 : legree of development of the glabella and superciliary arches (Fig. 1077). According 

 ;o Logan Turner, the dimensions of an average-sized sinus are: height, 1J in., from 

 J ;he lower end of the fronto-maxillary suture vertically upwards; breadth, 1 in., 

 'rom the median septum horizontally laterally; depth, f in., from the anterior wall 

 Backwards along the orbital roof. The sinus may exist merely as recesses limited 

 io a small area of bone above the nose, or it may extend upwards on to the fore- 

 lead for more than two inches ; laterally it may be limited by the bony wall of 

 ihe temporal fossa, while posteriorly it may reach as far back as the optic foramen. 

 Che anterior wall is thickest, but the thickness may vary from 1 to 5 mm. 

 The floor is the thinnest wall, hence when pus is retained within the cavity, 

 i t tends to point at the superior and medial angle of the orbit. Intra-cranial sup- 

 puration may arise in connexion with sinus disease by extension through the roof 



