12 



SUPERFICIAL ANATOMY OF THE HEAD AND NECK. 



the auditory meatus. Posteriorly, it approaches the descending part of the lateral 

 sinus, in some cases reaching close to the osseous lamina which forms the floor of 

 the groove, but more commonly the two are separated by an interval of from 5 to 

 10 mm. occupied by mastoid cells. The sinus is usually nearer to the surface than 

 the air-space. It will be remembered that the outer wall of the antrum is developed 

 from the postauditory process of the squamo-zygomatic division of the temporal 

 bone (see Osteology, p. 74) ; and there are generally in the adult some vestiges of 

 the infantile masto-squamosal suture in the form of small clefts and canals which 

 lead from the cavity to the exterior of the bone, and are occupied by connective 

 tissue and veins. 



remains of y' 

 masto - scjuamosdl 

 Stature 



facial, ri'erue 



Fig. 8. LOWKR AND POSTERIOR PORTION OP RIGHT TEMPORAL BONE, SHOWING THE SUPRAMEATAL 

 TRIANGLE, COURSE OP THE FACIAL NERVE, &C. Natural size. (GJ. D. T.) 



The mastoid antrum may be reached from the exterior by perforating the bone 

 close to the upper and posterior part of the external auditory meatus. In this 

 region Macewen describes a suprameatal triangle, 1 which is bounded above by the 

 supramastoid crest, below and in front by the postero-superior quadrant of the outer 

 margin of the osseous meatus, and behind by a vertical line tangential to the hind- 

 most point of that opening. The surface of bone included in the triangle is usually 

 marked by a small depression the suprameatal fossa, which is separated from the 

 aperture of the meatus by a sharp prominent edge the suprameatal spine. The 

 perforation should be made within this area, at the site of, or close behind, the 

 suprameatal fossa, and be directed inwards and slightly fowards, following the 

 inclination of the external auditory meatus. The antrum will then be opened at its 

 fore part, at a depth from the surface varying generally from 1 to 14 mm. ; in 

 extreme cases, and especially as the result of disease, this distance may be reduced 

 to 3 mm., or increased to 18 mm., or even more. At the lower part of the entrance 

 into the antrum the inner wall of the cavity presents a slight bulging over the 

 external semicircular canal (fig. 6), which may be injured if the instrument is not 

 checked as soon as the cavity is reached : the distance of the wall of the canal from 

 the surface is mostly between 17 and 20 mm. (about three-quarters of an inch). 

 Just below and in front of this, on the inner side of the epitympanic recess, is the 

 arch of the facial nerve contained in its canal, the osseous wall of which is thin 



1 W. Macewen, Pyogenic Infective Diseases of the Brain and Spinal Cord, 1893, p. . 



