A CONTRIBUTION TO THE MECHANISM OF DESCENT AND FLEXION IN VERTEX PRESENTATIONS. BY EDWARD REYNOLDS, M.D., Boston, Mass. [Reprinted from the American Journal of Obstetrics and Diseases of Women and Children, Vol. XX., April, 1887.] NEW YORK: WILLIAM WOOD & Cd., PUBLISHERS, 56 & 58 Lafayette Place. 1887. A CONTRIBUTION TO THE MECHANISM OF DESCENT IND FLEIION IN VERTEX PRESENTATIONS. BY EDWARD REYNOLDS, M.D., Boston, Mass. [Reprinted from the American Journal of Obstetrics and Diseases of Women and Children, Vol. XX., April, 1887.] NEW YORK: WILLIAM WOOD & CO., PUBLISHERS, 56 & 58 Lafayette Place. 1887. Pages 1-6 missing Flexion in Vertex Presentations. 7 DESCENT. On considering the mechanism of descent, we find that in the ordinary course of labor the os becomes considerably dilated before descent begins, that after the os is tolerably well dilated, the membranes rupture; the head then settles down against the os, driven by the pressure of the fluid behind it, and the presenting segment being then no longer subjected to the intrauterine pressure, and opposed only by the resistances of the parturient canal, descent begins under the driving force of the uterine fluid pressure behind. This may be called the normal mechanism of descent. There are other cases, however, in which no such timely rupture occurs, but in which the membranes remain intact until late in labor. In these cases, after a longer or shorter period of delay, the head advances to a point at which it rests against the os, and by a ball-valve action, cuts off the connection between the " fore-waters " and the uterine cavity; the presenting segment is then no longer exposed to the increased intrauterine pres- sure during the pains, and descent therefore goes on, much as though the membranes had broken. The degree of descent necessary for this " cutting off of the fore-waters " varies in different cases from a mere passage of the superior strait to complete descent even to the perineum, according to the vary- ing dilatability of the os and size of the head. It is only with this preliminary descent, previous to the " cut- ting off of the fore-waters," that this paper is concerned. It is generally admitted that, until the membranes have rup- tured, the contractions of the uterus act upon the ovum as a whole; but, I think, the corollary to this proposition, that dur- ing this time the fetus receives absolutely no motive impulse from the fluid medium by which it is surrounded, has not been sufficiently insisted upon. As was said above, it is a well-known law in physics, that if a solid body be immersed in fluid and exposed to pressure, the force exerted against it in any given direction is equal to that exerted in the opposite direction, and again that, if two forces be opposite and equal, no motion can result from their activity. Now the condition of the solid body named in these laws is precisely the condition of the fetus so long as the membranes Reynolds : The Mechanism of Descent and 8 remain unruptured and the " fore-waters " are not cut off, as is shown diagramatically in Fig. 3, where the forces marked A are opposed by the equal and opposite forces B, and can therefore have no share in the production of descent. It is to be remembered, however, that the fetus is still sub- jected to the influence of gravity (in the ordinary position of the mother), and it is certainly conceivable that, in cases of easy adaptation, the head may, under the influence of gravity A A a A B Fig. 3. alone, settle low enough to come in contact with the os, and by a ball-valve action " cut off the fore-waters " and so bring the fluid pressure into play. In the greater number of cases, however, the force of gravity must be powerless to overcome the resistance of the superior strait and thus effect the engagement of the head, and for these cases we must find some other explanation to account for the descent of the head. Lahs has argued that contact between the breech and fundus can never occur in the presence of a normal amount of liquor Flexion in Vertex Presentations. 9 amnii; but, with ah respect for his high authority, I think it can be shown that in these cases such contact not only can, but must occur, for as the membranes emerge from the os and begin to distend the vagina, a considerable quantity of liquor amnii is withdrawn from the uterine cavity by their expansion, this diminution of the uterine contents must be followed by retrac- tion, and if thus the long axis of the uterus is shortened while the breech is held, by the arrest of the head, at its original ele- vation in the abdomen, contact between breech and fundus must soon occur, and the contractile force of all the longitudinal fibres of the uterus is thus concentrated upon the breech. So soon as this force has produced sufficient descent to cut off the fore-waters, the influence of the fluid pressure is brought into play and descent goes on as though rupture had occurred. It is to be remembered, by the way, that though the force of direct contact is undoubtedly sufficient to drive a normal head through a normal inlet, it is much less powerful than the fluid pressure, since the one is produced by the contractions of the longitudinal fibres alone, while the other is due to the exertions of the uterus as a whole; a fact which explains the longer course of dry labors and labors with unbroken membranes. DESCENT AND FLEXION BEFORE LABOR. There remain for consideration those cases in which the head, in a state of more or less complete flexion, enters the pelvic cavity and may even proceed to the inferior strait during the latter months of pregnancy and before the advent of labor. The mechanism of these cases is similar to those just described, with the substitution of the tonic tension of the abdominal walls for the contractions of the uterus. Thus, if the intra-abdominal pressure be sufficiently strong, the lower segment of the uterus with a portion of the liquor amnii will be urged in the direction of least resistance into the pelvic cavity, the uterus as a whole descending, while the fetus is arrested by the contact of the head with the pelvic brim. It is evident that no great advance of the uterus can take place without the occurrence of contact between the fundus and the breech ; but so soon as this exists, the intra-abdominal pressure is directly transmitted to the breech, and if it be sufficiently strong will cause flexion and descent.