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Table of Contents

THE PELVIS.

Ossa innominata.—Sacrum.—Coccyx.—Distinction between the male and female pelvis.—Diameters of the pelvis.—Pelvis before puberty.—Axes.—Inclination.

The Pelvis, as the frame-work which, in great measure, contains, supports, and protects, the complicated apparatus of the generative organs, first claims our attention; since an accurate knowledge of the form, size, and uses, of its different parts is indispensably necessary, not only to understand the situation of the viscera it contains, but also to form a correct view of the mechanism upon which the process of parturition depends.

This osseous canal or circular archway, consists essentially of three bones, the right and left os innominatum, which form the sides of the arch, with the sacrum between them, acting as a keystone, and supporting the whole weight of the trunk above.

Ossa innominata. The ossa innominata in early life consists of three distinct bones, the iliac or hip bones at the sides, the ischia or lower portion upon which we sit, and the ossa pubis which meet each other anteriorly to form the front part of the pelvis. In the adult these are consolidated into one bone, merely leaving irregular lines and ridges here and there to mark their previous existence.

These bones present several striking points of resemblance with those which belong to the upper extremities, viz. the scapula and clavicle; and in the early stages of development, this similarity is much more distinctly seen: it is remarkable, that although the ischia and ossa pubis are formed later than the ilia, yet they unite with each other much sooner than with the ilia, so that the two consolidated bones bear the same relation to the ilium which is separated from them, that the clavicle does to the scapula: many other points of resemblance between the bones of the shoulder and pelvis might be noticed if necessary. (Meckel, Anat. vol. ii. p. 239.) The ossa innominata meet each other in front, forming the symphysis pubis, having layers of fibro-cartilage interposed between their extremities, and bound together by ligamentous fibres constituting the ligamentum arcuatum, or annulare ossium pubis, and by which a more rounded appearance is given to the pubic arch. They are united to the sacrum posteriorly, one on each side of it, forming the right and left sacro-iliac symphysis or synchondrosis; this differs in many respects from the symphysis pubis, the cartilaginous coverings of the opposing bones being much thinner, especially those of the ossa innominata; the surfaces are extremely uneven from the deep indentations which each bone presents at this part, locking, as it were, into each other, and thus contributing greatly to increase the firmness of the joint, which is also still farther strengthened by the support of powerful ligaments.

Between the ligamento-and cartilaginous layers which cover the surfaces of the bones at the pubic and sacro-iliac symphyses, a minute collection of synovial fluid may be detected, like that found in the fibro-cartilages between the vertebræ; it serves to lubricate their surfaces, and separates them more or less, thereby increasing the thickness of the intervening cartilaginous structure; and separating also the edges of the bones, to a certain extent, more especially at the symphysis pubis. (Portal, Anat. Méd.) These laminæ of intervening fibro-cartilage are thicker in the female than in the male, although of smaller extent; and this is still more remarkable during pregnancy, this ligamento-cartilaginous structure becoming now more cushiony and elastic, while in the latter months we can easily distinguish blood-vessels ramifying through it, which are branches of the pudic arteries and veins.

Sacrum. The sacrum, which forms the upper and posterior portion of the pelvis, contributes greatly to the general solidity of the whole bony circle. From its wedge-like shape, it is admirably adapted to support the entire weight of the trunk, and acts, as we have before observed, as a kind of keystone to the arch which is formed by the ossa innominata. It is of a triangular shape, being concave before and convex behind. In the fœtus it consists of five distinct pieces of bone separated by intervening layers of cartilage, like the vertebræ of the spinal column, and from their resemblance to those bones they have been called false vertebræ. These cartilages, after a time, gradually disappear; bony matter is deposited in their place; so that by the period of puberty the five sacral vertebræ become united into one solid bone, although they may be distinguished, until an advanced period of life, by the ridges which their edges form.

The upper surface of the sacrum, having to sustain the whole weight of the spinal column, is broad and flat, and corresponds to the lower surface of the last lumbar vertebra. Its anterior surface forms with that of the other mentioned bone a considerable angle, which projects forwards and more or less downwards towards the symphysis pubis, and is called the promontory of the sacrum. Beneath this point, the sacrum takes a considerable sweep backwards as it descends, gradually advancing again forwards, as we approach its inferior extremity, forming an extensive concavity upon its anterior surface: this is termed the hollow of the sacrum.

Coccyx. The lower end is prolonged by a small bone, called Coccyx or os Coccygis, from its supposed resemblance to a cuckoo’s beak. It usually consists of four, and sometimes (especially in women) of five portions; they are much smaller than the bones of the sacrum, and are very imperfect rudiments of vertebral formation; like these, they are at an early period little else than cartilage, and even when the bones are fully formed, they are united by intermediate cartilage, and thus retain so much mobility upon each other, as well as upon the lower end of the sacrum, as to admit of being forced backwards to the extent of a full inch, thus contributing greatly to increase the capacity of the outlet.

The sacrum not only serves to form the posterior parietes of the pelvis, but by the curve which its lower portion takes forwards, together with the coccyx, it gives a powerful support to the pelvic viscera.

When we take a general view of the bones which collectively form the pelvis, we find that it is evidently divided into two portions—an upper and a lower one. On the Continent these have been called the large and the small pelvis; in Britain we merely speak of the pelvis above or below the brim, the line of demarcation being the linea ilio-pectinea at the sides, the crista of the os pubis in front, and the promontory of the sacrum behind. The alæ of the ilia form a prominent feature in the upper pelvis, and not only afford an attachment for numerous muscles, but furnish a powerful and ample means of protection and support to the pelvic and lower abdominal viscera. In the female pelvis this is remarkably the case, the cavitas iliaca being well expanded and of greater extent than in the male, the crista of the ilium thrown more outwards; hence the distance between the antero-superior processes is much more considerable.

Distinction between the male and female pelvis. At the brim, the female pelvis presents several well-marked points of distinction from that of the male. The male pelvis has a contracted brim of a rounded or rather triangular form, with the promontory of the sacrum considerably projecting; whereas, that of the female is spacious, of an oval shape, and with a slightly prominent sacrum, thus affording more room for the passage of the child through the brim. The cavity of the male pelvis is deep, while in the female pelvis it is shallow, a circumstance which is very strikingly seen in comparing the length of the symphysis pubis in each, that of the male pelvis being nearly double the length of the female. This is an important point of difference as regards parturition, because in a shallow pelvis, the extent of surface exposed to the pressure of the head will be much less than where it is deep, and hence the resistance to the passage of the child will be proportionably diminished: in confirmation of this, we find that tall women, in whom the pelvis is usually deep, do not, on the whole, bear children so easily as women of middling stature in whom the pelvis is more shallow. The capacious hollow of the sacrum in the female pelvis adds also greatly to the extent of its cavity, and peculiarly adapts it for parturition, the injurious pressure of the head upon the soft linings of the pelvis being thus prevented, and every facility afforded for its quick and easy transit through the cavity. This applies especially to the neck of the bladder, which would almost inevitably suffer in every labour, were it not for the ample hollow of the sacrum relieving the pressure of the head against the anterior portions of the pelvis. The bones of the female pelvis being more slender and delicately formed, the foramina ovalia and sacro-ischiatic notches are wider, and thus add still farther to the capacity of the cavity.

In no part of the pelvis is the difference between the sexes more strongly marked than at the outlet. The spacious and well-rounded arch of the pubes in the female of the slender rami, is a striking contrast to the contracted angular arch of the male pelvis; and the tuberosities of the ischium being much wider apart, the head is enabled to pass under the arch with greater facility, and thus still farther to relieve the anterior of the pelvis from its pressure. The length of the sacro-sciatic ligaments, and the mobility of the coccyx upon the sacrum, by which it can be forced backwards to the extent of an inch by the pressure of the head during labour, not merely serve to distinguish it from the male pelvis, but afford a beautiful instance of design and adaptation.

The greater width of the pubic arch in the female pelvis is seen by comparing its angle with that of the arch in the male pelvis. In the female it has been estimated to form an angle varying between 90° and 100°, whereas in the male it is not more than between 70° and 80°. (Osiander, Handbuch der Embindungs-kunst, cap. iv. p. 58.)

From the greater width of the female pelvis, the acetubula are farther apart, and the great trochanters of the thigh-bones more projecting; hence the greater motion of the hips in the female when she walks, which is still more visible when she runs, for the motion is communicated to the whole trunk, so that each shoulder is turned more or less forwards as the corresponding foot is advanced. The thigh-bones, which are so far apart at their upper extremities, approach each other at the knees, contributing to produce that unsteady gait which is peculiar to the sex. “The woman,” says Mr. John Bell, “even of the most beautiful form, walks with a delicacy and feebleness which we come to acknowledge as a beauty in the weaker sex.” (Bell’s Anat. vol. i.)

These characteristic marks of the female figure, upon which its beauty in great measure depends, are well seen in all great works of art, whether of sculpture or painting. “The ancients,” as Mr. Abernethy has observed, “who had a clear and strong perception of whatever is beautiful or useful in the human figure, and who, perhaps, delicately exaggerated beauty to render it more striking, have represented Venus as measuring one-third more across the hips than the shoulders, whilst, in Apollo, they have reversed these measurements.” (Physiological Lectures.)

Diameters of the pelvis. It is of the utmost importance to the obstetrician, that he should be thoroughly acquainted with the various dimensions of the female pelvis, for, without this, he can form no correct idea of the manner in which the presenting part of the child passes through its brim, cavity and outlet during labour; indeed, unless he be thoroughly versed in this necessary point of obstetric knowledge, he will remain in almost total ignorance of the whole mechanism of parturition, which must, in great measure, be looked upon as the basis of practical midwifery. The dimensions of the brim cavity and outlet of the pelvis may be given with sufficient correctness for all practical purposes, by measuring three of their diameters,—1. the straight, antero-posterior, or conjugate; 2. the transverse; and 3. the oblique. At the brim they are as follow:—the straight diameter, drawn from the middle of the promontorium sacri to the upper edge of the symphisis pubis, 4·3 inches; the transverse diameter, from the middle of the linea-ilio-pectinea of one ilium to that of the other, 5·4 inches; and the oblique diameter, from one sacro-iliac synchondrosis to the opposite acetabulum, 4·8 inches. The oblique diameters are called right and left, according to the sacro-iliac symphysis from which they are drawn.

Fig. 1. Fig. 2.