Mechanism of the second stage of human parturition

Abstract

The mechanism of the second stage of human parturition has already been the subject of numerous works, and on many aspects of the problem the treatment has been exhaustive. There are matters, however which still call for inquiry. Further evidence seems desirable relative to the form of the dilated pelvic canal; the direction of uterine pressure may be said to be unknown: given a variable direction of pressure proof is awanting of its influence, or otherwise, on the mechanism; the known effect of a uniform pressure on a curved surface needs application to the mechanism of labour: it is worth while to attempt to unravel the congeries of movements which go by the names of flexion and extension; the difficult problem bf lateral obliquities of the fetal head remains where it was forty years ago; the development of the mechanism at the vulvar outlet may be capable of modification in a manner favourable to easier birth. These and other matters are gone over in the pages that follow.A definition of what is meant by the second stage is not easy if regard is had to every possibility/ possibility. In most labours the second stage comprehends the period which elapses after the os uteri is fully dilated and the membranes are ruptured until the child is completely born. It is obvious, however, that the mechanical phenomena cf the second stage are able to be developed before the os is wide open, and after full dilation before the membranes are ruptured, at any rate until a late period of the second staged The "expulsïve period" of the Germans also is not sufficiently general at least in its clinical manifestations, for the evidence - abdominal action - may be awantingd Nor is it adequate to maintain that the passage of the individual segments of the fetus through the vaginal and vulvar canals constitutes the second stage. For the present purpose the second stage is defined as a period which begins when the fetus is compelled to exercise an uncertain quantity of pressure upon the upper part of the vaginal wall and which lasts until, the child is entirely born.The present thesis is divided into six sections and two appendices. The first section deals with the form of the pelvic canal, the second treats of the direction of uterine and abdominal pressure, in the third the mechanism of flexion is considered in the fourth lateral obliquity of the fetal head, the fifth is devoted to internal rotation; while the sixth is concerned with the mechanism of extension and the protection of the perineum. In the first appendix a number of simple experiments is described, and in the second are set forth tables of angular distances derived from the published reproductions of frozen sections. Finally a short summary is given of the results embodied in the thesis.