TACT PRESENTATIONS.
The average frequency thus varies, according to different authors, from 1 in 147 (minimum) to 1 in 247 (maximum). As for the vertex, we ad mit two chief positions, M.I.R. and M.I.L., with their varieties, viz., anterior, transverse and posterior. The position M.I.R. seems to be more frequent than M.I.L.; according to the following statistics of Madame Lachapelle and Depaul. M. I. R. M. I. L.
Madame Lachapelle, 41 31 Depaul, 57 23 Naegele states that the face presents always in the left oblique diame ter, i.e., in M.I.R.P. or in M.I.L.A. This opinion seems to us to be far too absolute. Our personal observations justify us in concluding that the oblique presentation of the face is much less frequent than the transverse, and that, particularly in original face presentations, the transverse presentation M.I.R. is much more frequent than M.I.L. Al though the transverse position seems more frequent in original face pres entations, it is not so with secondary presentations. The secondary pres
entations follow vertex presentations, and, since the presentation 0.L.A. is much the most frequent, its transformation into a face case almost necessarily produces a M.I.R. P. The vertex presentation O. R. P. likewise transforms itself into M.I.L. A., and although Naegele is right in saying that the chin is almost always turned backward in right positions, he is only right in regard to secondary face presentations. As enunciated by himself, his proposition is too absolute.
We will first study the character of face presentations in general, and will then take the most frequent position, viz., M.I.R.P. to represent the mechanism of labor.... As stated in the article on Presentation, we take as a landmark on the head of the foetus, not the forehead, as do certain writers, but the chin, which must necessarily rotate beneath the symphy- sis in order that labor may be completed.