The special cause of this deformity, when not dependent upon the infantile, masculine, or funnel-shaped pelvis before described, is to be attributed to tbe action of the great glutei muscles, in standing and walking, pressing upon the ischia partially softened by disease.
Contraction of the antero-posterior diameter.
This results in most cases from ankylosis of the sacro-coccygeal joint. The coccyx is rendered immoveable and incapable of yielding to the head of the fcetus, so as to bring about the usual increase in the antero-posterior diameter. In addition, it is usually ankylosed in an almost horizontal direction, with its apex directed forwards and its surfaces upwards and downwards, a position which is brought about by the resiliency of the sacro-sciatic liga ments and ischio-coccygeus muscles, and by a continued sitting posture ; —and thus the an. tero-posterior diameter is still more diminished.
The fcetal head is arrested by, and rests on, the coccyx, and tbe obstruction is only over come by the giving way of the bone at or near the ankylosed part. The delivery gener ally requires instrumental aid.
Examples of this condition are nume rous. In the practice of Dr. Blichaelis of Kiel, a fracture of the parietal bone of the infant was occasioned by an immobility of the coccyx resulting from ankylosis.* Dr. Merriman, in a Letter to Dr. Lee, published in the Med. Gazette (1813, p. 221.), mentions a case in which the point of the coccyx snapped off in three successive labours, and he had observed one or two other cases in which this occurred, and no ill consequences followed. In one, the coccyx was turned upwards, and there was a considerable bulk of ankylosis, pro duced by a fall. Dr. F. Ramsbotbam also mentions three instances in which fracture at or near an ankylosis of the coccyx took place. Dr. Lee relates also a case which occurred in the praetice of a country surgeon, in which the lower end of the sacrum curved much forward and was ankylosed to the upper coccygeal bones, the last only being moveable.
In certain cases the contraction of this diameter is brought about by the liking forward of the lower end of the sacrum, with an almost horizontal direction of the lower part of the sacral curve. I have met with two or three
pelves in which this condition is present, coexistent with, and evidently caused by, a curvature of the lower dorsal region of the spinal column backwards. By such a curvature, the line of gravity of the trunk is displaced back wards, passing through the sacrum consider ably behind its base, and making traction upon it in that direction. The promontory of the sacrum is by this means dragged backwards and upwards, and the conjugate diameter of the brim increased, so as to be equal to, or greater than, the transverse. The shape of the brim is thus rendered oblong or oval in the opposite direction to, and contrasting with, the ovate deformity presently to be described. The axis of the superior plane is rendered more horizontal than normal, by the increase of the pelvi-vertebral angle, the cotylo sacral arch more open in its curve, and the acetabula in some measure approximated. The cavity of the pelvis is narrowed antero-pos teriorly by the gradual advance downwards of the posterior wall, and the vertical curvature vertebrm (a), directed backwards to a great extent, and somewhat to the left side, with a compensatory curve to the right, at the junc tion of the lumbar with the dorsal vertebrm (b). The lower lumbar vertebrm and sacral promontory. are twisted considerably towards the left side, and dragged backwards, the sacral promontory being also considerably raised up wards (c). The pelvis is of exceedingly large general capacity, and otherwise well formed, showing evidently, by the preponderance of the conjugate diameter and appearance of the sacrum, the effect of the deformed spine upon it. The conjugate diameter of the brim measures as much as 5 and a half inches ; the transverse 5+ inches. At the outlet, the distance between the tuberosities is only inches, and the sub-pubic angle 75° ; but, from an unfortunate deficiency of the lower end of the sacrum, the antero-posterior diameter cannot be measured.