The two lateral projections, the tuberosities of the ischia, extend somewhat beneath the extremity of the coccyx, and being unyielding, alone support the entire weight of the body in the sitting posture. Hence it happens that the transverse contractions of the pelvis are more common at the inferior strait, than the antero-posterior. The deep notch found anteriorly on the inferior strait, corresponds to the summit or inferior margin of the arch of the pubis. This notch forms an important feature in obstetric practice, and it is much broader in the female pelvis than in the male. At its base this arch is from three and a half to three and a quarter inches broad, but only from one and a quarter to one and a half inches wide at its apex; its height is from two to two and a half inches. As will be afterwards noticed, in describing the process of parturition, this notch has very important relations to the foetal head, affording to it a more ready exit in the first position.
The still deeper notches, which appear posteriorly on each side of the coccyx, are subtended by the sciatic ligaments, whose elasticity is capable of materially adding to the pelvic dimensions at the moment of the final passage of the foetal head. By the combination of double arches, both anteriorly and posteriorly, and from above downward, the greatest possible strength has been preserved to the pelvis, without too much increasing its weight; while at the same time the head is enabled all the sooner to emerge from its bony prison, by reason of these three important notches or arches in the periphery of the inferior strait. While the anterior notch, that of the pubic arch,- also affords inestimable advantages to the accoucheur, in the introduction of the hand, the forceps or other instruments, where, from irregularity or contraction of the pelvis, from abnormal size of the head, or from other reasons, manual or mechanical interference becomes indispensable.
The plane of the inferior strait may be demonstrated by applying a sheet of paper in the following manner. Upon an inverted pelvis, in which the sacro-coccygeal articulation is still flexible, and in which the coccyx is retracted as far as possible, we place a piece of paper narrow enough to pass between the tuberosities of the ischium and be closely fitted to the summit of the public arch. This paper, thus resting in front upon the inverted surface of the arch of the pubis, and posteriorly upon the retracted extremity of the coccyx, will accurately represent the proper plane of the inferior strait. Hence it will be at once obvious that this plane is by no means parallel with that of the superior strait. The angle of inclination of the latter being from fifty-five to sixty degrees, and that of the former but from ten to eleven degrees. And as the planes of the two straits are not parallel, so neither will their axes be in the same line.
Thus, as is shown in Fig. 3, the plane of the inferior strait with the coccyx retracted as it is in the last stage of labor, – the only stage in which either the coccyx or in fact the inferior strait itself is particularly involved,-inclines backwards and downwards at a very small angle below the horizontal line. While, as shown in Fig. 1, the plane of the inferior strait in the ordinary condition of the pelvis,- with the coccyx not retracted, -inches at a correspondingly small angle above the horizontal line. And the prolongation of the planes of the superior and inferior straits, to their point of union beyond the pubis, shows that, instead of being parallel with each other in the human pelvis, they form an angle of not far from 45*; especially when the point of the coccyx is pushed back, as it is by the foetal head in its passage through the inferior strait.
The great advantage which must arise from this apparently irregular arrangement of the pelvic planes and axes, will be evident from a moment’s consideration of what must otherwise be the effect of gravity in the upright position, especially in the advanced stages of pregnancy. Even if the head and entire foetus were not impacted in the cavity of the pelvis, premature delivery must necessarily result from the constant and directly downward tendency of the uterus and its content. This is still further illustrated by comparison with the almost perfect parallelism which appears in the planes of the superior and inferior pelvic straits of quadrupeds. Here the gravid uterus is entirely supported by the abdominal parietes; and the planes of the superior and inferior,-or more accurately speaking, anterior and posterior, -straits, are nearly parallel; and their corresponding axes nearly identical with each other and parallel with the trunk itself.
The axis of the inferior strait is the line which, drawn perpendicular to its plane, falls upon it midway between the symphysis pubis and the extremity of the coccyx. When the coccyx is in its usual position, this line extended, will strike the promontory of the sacrum; but when the coccyx is retracted, as in the advanced stages of labor, this line will fall upon the articulation of the first with the second bone of the sacrum. The line g, f, Fig. 1, which represents the axis of the inferior strait, in the ordinary, unretracted condition of the coccyx, should have been made to strike the sacro-vertebral angle; then it would have been perpendicular to the plane. The axis of the inferior strait forms the same angle with that of the superior strait, that the plane of the inferior strait forms with the corresponding plane of the superior strait.
The diameter most important to be observed in the inferior strait, correspond in number and in name to those of the superior strait. First, the diameter or coccy-pubal diameter,- extending from the extremity of the coccyx to the summit or inferior surface of the pubic arch,-measure five inches when it is not thus retracted. For practical purposes, therefore, the antero-posterior diameter of the inferior strait.-taken at any period of labor when the head has not so engaged in that strait as to push back the coccyx,-must be considered as half an inch longer than the measure shows. The only exception to this will be, in those cases in which from complete ossification of the coccygeal and sacro-coccygeal articulations, the coccyx forms one continuous curve and one solid bone with the sacrum.
Fig. 3.
c.d. The horizontal line. c.e. The plane of the inferior strait (during labor). a.b. The axis of the inferior strait.
The second diameter of the inferior strait, is the transverse or bi-ischiatic. This extends from the inner margin of the tuberosity of the ischium on one side, to the corresponding point on the other side; and measures four inches.
The third diameter of the inferior strait, is the oblique. This extends from the point of junction of the rami of the pubis and ischium, to the middle of the great sacro-sciatic ligament on the opposite side; and measures four inches. As previously stated, from the distension of the ligamentous portions of the pelvic parietes, the capacity of the inferior strait may be somewhat enlarged in this direction. And some authorities estimate this oblique diameter as practically equal therefore to four and a half inches. This diameter again, may be either the right or the left oblique. But it is usually considered as one diameter; because, except in some not very common forms of pelvic distortion, the one will be exactly equal to the other. And where there is reason to suspect distortion, it will be proper therefore to measure the diameter on each side.
THE CAVITY OF THE PELVIS.-That part of the canal of the pelvis which occupies the space between the superior and the inferior straits, is known as the cavity of the pelvis. From its situation and consequence relation to the uterus and ovum, it will be obvious that a thorough knowledge of this cavity, of its axes, planes and dimensions, is not only important in all ordinary cases of labor, but indispensably necessary to the successful management of all those of serious difficulty. This cavity is irregular in shape; contracted in its superior and inferior orifices; and greatly enlarged posteriorly by the concavity of the sacrum. The depth of the pelvis at the symphysis pubis in front, is but one inch, and a half; at the side, from the tuberosity of the ischium to the linea ilia, it is three inches and a half; and posteriorly, from the promontory of the sacrum to the extremity of the coccyx, the depth is four and three-fourths inches, or five inches,- according as the coccyx is in its natural position, or retracted as in labor.
Fig. 4.
A.A. The antero-posterior or coccy-pubal diameter. B.B. The transverse or bis-ischiatic diameter. C.C. The two oblique diameters.
The diameters of the pelvic cavity, or centre of the excavation, correspond in number and in name to those of the superior and inferior straits; and they are all of very nearly the same length of four inches and three-fourths. The antero-posterior diameter, extending from the sub-pubic ligament,-that is, from the summit or under surface of the pubic arch to the middle of the sacrum,- measures four inches and three quarters. But the line drawn from the sub-pubic ligament to the promontory of the sacrum, will be found to measure but four inches and a half. The transverse and oblique diameters are each four inches and three-fourths in length. It should be borne in mind that the long diameter of the superior strait is the transverse; but that the antero-posterior is the long diameter of the inferior strait.
