15 lo HUMAN ANATOMY. 



mm. below the level of its upper wall. If the sinus is in an abnormally anterior posi- 

 tion, the posterior wall of the meatus must be removed to gain more room. 



The facial nerve is also in great danger in these operations, and has frequently 

 been injured. It lies in the inner wall of the mouth of the antrum, and is therefore, 

 in front of it. The antrum is approximately about 12 mm. (one-half inch) in a direc- 

 tion very slightly inward, forward, and upward from a point on the external surface, 

 5 mm. posterior to the suprameatal spine. The anterior edge of the opening made 

 to reach the antrum should be at this point, and its upper edge 3 mm. below the 

 spine. It should never be carried deeper than i^ cm. ( S^4 in.) from the anterior 

 edge of the external opening, for fear of injuring the facial nerve or external semi- 

 circular canal. 



As the situation of the mastoid antrum is the key to the position in all operations 

 upon either the antrum itself or the mastoid cells, Macewen has noted three points in 

 the anatomy of the mastoid that may govern the surgeon in reaching the antrum 

 without (a) opening the sigmoid groove and injuring its enclosed sinus; (d) 

 encroaching upon the Fallopian canal and destroying the facial nerve ; (c) invading 

 the middle cerebral fossa ; (d) injuring the semicircular canals. 



1. The suprameatal triangle — the lower border of which corresponds with 

 the level of the roof of the antrum, and is, therefore, a few lines below the level of the 

 base of the temporo-sphenoidal lobe — is bounded above by the posterior root of the 

 zygoma, below by the postero-superior segment of the bony external meatus, and 

 behind by a line uniting these two and drawn vertically from the posterior border of 

 the meatus to the zygomatic root. The opening is made within this triangle and 

 close to the last line — the base of the triangle. 



2. The excavation of the bone is carried inward and a little forward, in the direc- 

 tion of the posterior wall of the bony meatus, as shown by a probe passed into it from 

 behind between the skin and the osseous wall. The more oblique the direction of 

 this wall from behind forward, the more anterior the situation of the antrum. 



3. The depth of the inner wall of the tympanic cavity from the level of the 

 skull at the bony external meatus should be determined by introducing a probe 

 through the external ear (and through the tympanic membrane previously per- 

 forated by pathological processes) until it touches the inner wall of the tympanum. 

 If this cavity lies deeply, the more superficial mastoid antrum will be relatively 

 deep also. 



Of forty brain abscesses, the bone was diseased directly to the dura in thirty-seven 

 (92 per cent.), the bone was diseased, but not the dura, in one (2.5 per cent.), and 

 the bone was healthy in two (5 per cent.) (Korner). 



It follows from this list of cases, that after a thorough exposure of the antrum and 

 the ear cavities, the carious process should be followed inward to the dura or brain. 

 In case an abscess in the temporo-sphenoidal lobe cannot be reached in this way the 

 skull may be opened by a trephine, or by an osteo-plastic resection immediately above 

 the ear. A cerebellar abscess might be reached by an opening one and one-half 

 inches behind the centre of the bony meatus and one inch below Reid's base line. 



THE INTERNAL EAR. 



The internal ear consists essentially of a highly complex membranous sac, con- 

 nected with the peripheral ramifications of the auditory nerve, and a bony capsule, 

 which encloses all parts of the membranous structure and is embedded within the 

 substance of the petrous portion of the temporal bone. These two parts, known 

 respectively as the membrano^is and the bony /abyfinth, are not everywhere in close 

 apposition, but in most places are separated by an intervening space filled with a 

 fluid, the perilymph, the inner sac lying within the osseous capsule like a shrunken 

 cast within a mould. The membranous labyrinth is hollow and everywhere filled 

 with a fluid, called the endolymph, which nowhere gains access to the cavity 

 occupied by the perilymph. The internal ear is closely related, on the one side, 

 with the bottom of the internal auditory canal, which its inner wall contributes, and 

 with the inner wall of the tympanic cavity on the other. Its entire length is about 

 20 mm. , and its long axis corresponds closely with that of the pyramidal or petrous 



