ARCHIVES OF OTOLOGY.

THE LIMITS OF VARIATION IN THE DEPTH OF THE MASTOID ANTRUM.'

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BY PHILIP D. KERRISON, M.D.

(With ten illustrations on Text-plates II.-V.)

HILE all observers agree that the antrum may be reached at a depth of 8 to 10 mm, there is a surprising diversity of opinion as to the maximum depth, or the depth beyond which it is not safe to proceed in an attempt to expose the antrum. The following authors are cited as to its maximum depth: Gruber, 15 mm (a little less than inch); Politzer, 15 mm; Buck, inch; Dench, inch; Schwartze, 25 mm (about 1 inch); Broca, 29 mm, or about 1 inches.

Between the two extremes here expressed the contradic. tion is even greater than at first appears; for whereas Gruber and Politzer give 15 mm as the depth beyond which it is not safe to go, Broca contends that if the antrum is not reached at a depth of 25 mm, the surgeon should not be afraid to go still farther. Obviously one or the other view must be incorrect.

To bring order out of this confusing diversity of opinion, some fixed point upon the mastoid cortex should be agreed upon, from which to measure the depth of the antrum. This, in the writer's opinion, should be the one point which is always nearest the antrum-viz., the space just behind the spine of Henle, and inclosed in the well-known triangle formed by the postero-superior arc of the meatus and the

Read before the Section on Otology of the N. Y. Academy of Medicine, March 12, 1903.

lines running tangent to the superior and posterior walls of the meatus respectively. This space we shall call for convenience the triangle of election.

As the antrum always lies immediately behind the tympanic vault, and as the postero-superior wall of the meatus always measures the distance between the vault and the cortical surface just behind the spine of Henle, it occurred to the writer that some relation might be found to exist between the length of the postero-superior wall of the meatus and the depth of the antrum.

To determine this, careful measurements were taken of thirty bones, taken at random as they could be obtained. In measuring the postero-superior wall of the meatus, the distance in millimetres was taken between Henle's spine externally and the inner margin of the meatus internally. Sections were then made, bisecting the mastoid cortex in a vertical line passing through the posterior boundary of the triangle of election, and cleaving the bone from before backward in a plane diverging from that of the posterior wall of the meatus by an angle of 30 to 35 degrees. These sections in every case exposed the antrum, and made it an easy matter to measure the thickness of the bone separating it from the mastoid cortex.

The measurements thus obtained seemed clearly to prove three facts: viz., Ist, that in different temporal bones much greater variations exist as to the length of the bony meatus than are noted in most text-books; 2d, that the depth of the antrum is always less by actual measurement than the postero-superior canal wall; and 3d, that the depth of the antrum rarely, if ever, exceeds 15 mm, or about inch. (See Text-plate No. II., Figs. 1 and 2.)

In the thirty bones presented for examination, the length of the postero-superior canal wall varies from 12 mm to 18 mm as follows:-in three it is 12 mm; in three, 13 mm; in eight it is 14 mm; in five, 15 mm; in eight 16 mm; in two 17 mm., and in one it is 18 mm.

In the same bones the depth of the antrum varies from 6 to 15 mm. These varying depths may be stated in their relation to the canal wall as follows: