Moulding of the Foetal Head: A COMPENSATORY MECHANISM.

IT has long been known to obstetricians that compression-force applied in the suboccipito bregmatic circumference of the foetal head results in some diminution of the related diameters and is accompanied by a compensatory movement of elongation in the rnento-vertical diameter. There is very little description of this mechanism in the literature, and this paper attempts to describe how this compensatory movement is brought about, together with its limitations in vertex presentation. The study of the bones and sutures of the vault can best be made by removing the intact vault still attached to a portion of the base. I t is clear that the mobility of any two bones along any suture line will depend largely on the breadth of the connecting membrane between their opposing edges. It is also clear that mobility will be something less than this breadth, for in the case of the lambdoidal , coronal and frontal sutures the breadth of the uniting tissue decreases as the base is approached, and where overlapping is actually produced the thickness of the bone edges must be another limiting factor. The maximum amount of movement along these different suture lines can be measured, and is found to take place at that point of any bone which is most remote from its base, and this is , of course , due to the decreasing breadth of the suture from apex to base. The interesting fact about the movements of these bones of the vault lies in the observation that while the upper Anatomical factors. portions of the frontal and occipital bones readily respond to forces which tend to depress them, they cannot be made to overlap easily by sliding one bone under the other, and unless very great force is used only slight overlapping is achieved. The impression gained from manipulation of the bones is one of great resistance to any sliding motion. Measurement shows the occipital bone to be capable of an inward movement of a inch (6 mm.) at its apex and an inward movement of equal magnitude occurs where each frontal bone reaches the lateral margin of the bregma. From the apex to the base of each bone the depression rapidly decreases. Overlap cannot be accurately measured, but would appear to be no more than 4 inch (3 mm.) at any point where it can be produced. It is sometimes impossible to make the parietal bones overlap along the sagittal suture except by the merest fraction of an inch or with the aid of such force as must rupture the membranous tissue. The reason why the depressing movement should be so much in excess of the sliding movement is possibly due to 3 factors. Firstly, a true sliding motion would be possible only if each bone were entirely mobile instead of being fixed at its base. Secondly, the manner in which the underlying sinuses are attached to the suture tissue tends to limit a sliding movement more readily than the movement of depression. This fact is easily demonstrated by dissecting the dural septum with its enclosed sinus from it; attachment along