Irregularities of the pelvi-vertebral angle.— Too great obliquity of the pelvis has been said to cause "ante-version," by throwing the weight of the uterus on the anterior abdomi nal walls ; and too little obliquity to have a tendency to produce prolapsus uteri. Naegele (Das weibliche Becken), however, considers many of these supposed consequences are theoretical only; but he observed, both in first, and after many labours, that an approximation of tbe pelvic plane to the perpendicular caused the fcetal head to be placed so much forward, and the os uteri so high, as to be felt with. difficulty ; and, on the other hand, in very considerable inclination to the horizon the fcetal head was deep, and not easy to be felt through the uterine neck. In neither condi tion was there any important deviation from the mechanism of natural parturition.
The alteration of the planes of the pelvic outlets becomes, however, a valuable means of indication of the more important class of pel vic distortions, in which such an alteration is generally effected. In particular, when the tip of the coccyx and pubic symphysis are un usually placed, especial attention to the form and measurements of the pelvis is called for ; though, as before seen, an alteration of the pelvic planes alone does not- necessarily imply a distorted pelvis.
Dr. Rigby mentions, that the pelvic inclina tion is generally less in a tall slender person than in a short thick-set woman ; and that, in the forrner case, the hollow of the sacrum is generally small, and in the latter deep.
2. Distortions of the pelvis are best arranged into the following practical divisions,"according as they affect the brim ;—the cavity ;—or the outlet only or principally ;—or the whole struc ture of the pelvis at the same time.* Distortions affecting the brim only or princi pally.,-By far the greater majority of these cases consist in an unusually forward projection of the sacral promontory. This causes the opening to assume a heart-shape, diminishes the conjugate diameter, sometimes contracting also the oblique, and increasing the transverse in some degree. At the same time, the pro montory of the sacrum is sunk down below its norrnal altitude, lessening the angle of the superior plane, and making its axis assume a more vertical direction. Most generally the sacral projection deviates from the medium line, and forms the lower extremity of an abnormal curve in the lumbar vertebra, render. ing the brim of the pelvis generally unsym metrical in shape.
This deviation takes place most frequently, according to my own observation, to the left side, and appears to be an exaggeration of a very common tendency (which, being seen in the most robust subjects, can scarcely be called abnormal) of the natural curve of the lumbar vertebra towards the left side. This lateral curve is evidently a compensatory one to the very usual and well-known deviation of the line of thoracic vertebra to the right side, and is calculated to keep the line of the whole spinal column in equivalent relation to the di rect and perpendicular line of gravity. It is an interesting question, how far this common ten dency of the lumbar curve influences the po sition of the ftal head, by affording more room for the sinciput at the right sacro-iliae joint, and determining its long axis in the left oblique diameter, which is generally allowed to be the most frequent presentation. In by far the greater number of all kinds of pelvic deformity that have come under my own ob servation, the projection of the sacral promon tory, when present, has been towards the left side, showing the effect of a natural and previ ously existing tendency when the supporting structures have been softened by disease. To this subject I shall presently have occasion again to advert.
In these cases of partial distortion of the pelvic brim, the cavity and inferior outlet are generally roomy, shallow, and open. Often the transverse or antero-posterior diameter of the inferior outlet is larger than natural, and the pelvis then approaches the condition of the complete ovate deformity. An important prac tical result of this fact is, that in these defor mities of the brim all operations for extraction of the fcetus " per vias naturates" are facilitated and rendered more successful. The obstruc tion being simply and only at the brim, when that is overcome the rest is easy.
Two examples of deformity of the briin are recorded by Dr. Ramsbotham. One is pre served in the Museum of the London Hospital (see fig. 114.). The conjugate diameter of its diameters, but more especially at the brim. The conjugate diameter is 21 inches only ; the transverse 1/ inches ; and while the dis tance from the right side of the sacral promon tory to the right pubic ramus is 2i inches, that on the left side is 2+ inches only. The depth of the cavity posteriorly is only 4 inches ; the spines of the ischia project considerably inwards, and the distance between the tubero sities measures only 31 inches.