THE TRUE PELVIS IN GENERAL.
The walls of the true pelvis are unequal in length. The posterior wall is from 5.4 to 6.2 inches; the anterior wall along its median line is only 1.5 to 1.9 inches, and the lateral wall about 3.9 inches. Its capacity varies in different parts. The lateral walls represent two oblique planes which approach each other as they descend, thus giving to the pelvis the form of a cone, base uppermost. As to the anterior and posterior walls, the mobility of the coccyx allows the inferior strait to exceed the superior strait in the antero-posterior direction. Finally, towards the middle of the cavity, the projections of the sciatic spines make the pelvis narrower and seem to form a median strait between the other two.
From these facts result the following, according to Pajot:— 1. The fcetus cannot enter the cavity and cross it unless it does not exceed 5.7 inches for the superior, and 4.6 inches for the inferior strait. Therefore it must reach the superior strait by the vertex, or the opposite end of the trunk.
2. If the head of the foetus enters the superior strait in dimensions equal to the transverse diameter, it is immediately stopped by the near ness of the walls before reaching the bottom of the true pelvis, and it cannot cross the inferior strait without diminishing in volume, or chang ing its direction. The result of this change would be the presentation of its greater dimensions in the direction of the diameter A P, or of one of the oblique diameters.
3. When the head of the fcetus has reached the bottom of the cavity, those of its parts which point backward are still applied to the posterior wall, while that which is in front will have already freed itself from the anterior wall, which is shorter, and corresponds to the pubic arch. It will then be free from the whole solid wall.
4. The foetus cannot enter and cross the pelvis unless it first follows the direction of the axis of the superior strait, then crosses the true pelvis following the curved line, which is its axis; and finally, in order to issue forth, it must follow the direction of the axis of the inferior strait.
The foetus must therefore bend and describe a curve with its concavity forward.
Differences in the Pelvis, according to the Individual, Sex, Age and Race. The pelvis shows remarkable differences according to the individual, sex, age and race.
I. Individual.—Among women of the same race the pelvis, without ceasing to be normal, shows numerous variations. The thickness, the solidity of the bones, the points of the crests and eminences, larger or smaller, the height, the size, the variable curvature of the sacrum, the angle of the pubic arch, the different heights of the ossa innominata, con stitute as many varieties. Yet with the younger Stein and Weber we dis tinguish four forms of the superior strait.
1st. The shape of the heart on a playing-card, with the point cut off. 2d. Elliptical, in which the transverse diameter is the greater.
3d. Round.
4th. Elliptical, in which the diameter A P is the greater.
II. pelvis of a woman differs greatly from that of a man. (Figs. 15 and 16). The following are these differences, according to Dr. Verneau: 1st. In a pelvis there are sexal differences of two kinds: the first, .analogous to those which are found in the rest of the skeleton, relate to the marks left by the muscular system; the second are peculiar to the pelvis.
2d. The real differences between the pelves are found almost exclusively in the true•pelvis, and are determined by the presence of the uterus.
3d, The form of the superior circumference is the same in both sexes, the ratio of the maximum of the antero-posterior to the maximum trans verse diameter is in both 0.62.
4th. All the dimensions of the internal iliac-fossa are less in woman, except the distance which separates the antero-superior iliac spine from the sacroiliac articulation.
5th. The internal iliac-fossa is shallower in the female.