But when the lateral pelvic arches are al ready impressed with the angular deformity, the sitting posture has the effect of merely increasing the inward bend, and approximat ing the acetabula and sc'atic tuberosities to the sacrum, pressed down by the superin cumbent weight. Dr. Rigby mentions that frequent riding on horseback at an early age will produce contraction of the inferior outlet, even in the healthy pelvis, and that the females of those American nations who ride much bear few children, and are often three or four days in severe labour.
In certain cases, in which the acetabulum on one sille only has been pressed inwards by the constant use of the lateral recumbent po sition, or in which the centre of gravity of the trunk has been permanently shifted to one side by the spinal bend, a habit is acquired of sitting more upon one ischial tuberosity thar the other. This unequal pressure produces inequality of distortion, aud presses the tu berosity and acetabulum of that side inwards, while the opposite one presents the usual di vergence of the elliptical distortion. This effect is also contributed to in like manner, under the same circunistances, in the standing position, by the pressure being greater and more frequent upon one feour than the other ; and thus we have produced a sort of oblique deformity, of which I have seen se veral specimens.
lit the Museum of King,'s College are three skeletons, all presenting more or less a ten dency to this peculiar modification of the ovate deformity. In the Hunterian Museum is another, in an adult female skeleton, still more marked.
It is somewhat remarkable that, in all these examples, the trunk is bent towards the right side,and the lumbar curve and sacral projection towards the left; so that the line of gravity, and consequently the greatest share of supporting the weight of the body, falls nearer the left leg and the left side of the pelvis. The effect is such as to produce great similarity in the form of all these pelves, which vary only in the de gree of distortion. The sacral promontory is directed to the left side, while the sacral con cavity is more or less twisted so as to face the left acetabulum. The innominate bone of the left side is placed lower and more ver tical than that of the right side, which appears longer and less bent ; so that the left ischial tuberosity projects lower and more vertically than the right, which is everted and directed outwards. The left acetabulum is brought
nearer to, and rnore directly under, the sacral promontory, the cotylo-sacral arch being more curved than the right ; while the right sacro iliac joint and lateral sacral mass are higher, the cotylo - sacral curve more open, the iliac wing more spread, and directed, like the acetabulum, more forwards, and the ischio pubic ramus placed more obliquely, than those on the left side.
In the female Hunterian skeleton, the ob liquity of the spine and pelvis are so great, that the upper dorsal vertebrx are placed above the right sacro-iliac joint. The femora are shortened, and curved forwards and out wards, and the leg bones forwards and inwards, in compensating curves. The left knee, how ever, is more under the line of gravity than the right. A tendency to a somewhat similar twist is seen in an adult hydrocephalic ske leton in the same collection.
These pelves present, at first sight, some re semblance to the very different " obliquely ovate" pelvis of Naegele. The most charac teristic differences are, the presence of other rickety appearances, and the want of the co incidence of lateral deviation of the pubic symphysis with the sacro-iliac ankylosis and malformation of the latter.
Rokitansky includes all the pelves which present a want of symmetry at the sides under the general term of oblique pelves, after Osian der's classification, in which he comprehends by far the greatest number of pelvic deformi ties of all kinds. He gives, as a characteristic of this class,—approximation of the sacral promontory to the pectineal eminence on one side, which side has also a higher level and a less pelvic inclination than the opposite one, originating in a lateral curve and torsion of the sacrum towards the contracted side, —and straightening out of the linea innominata on the opposite side, between the sacro-iliac joint and the acetabulum. It includes the frequent pelvic deviations resulting from lateral curva ture of the spine, but most frequently arises from rickets, or displacement of the femur by hip-joint disease or violence.