Ligaments of Coccyx.-The sacrum is joined to the coccyx, and, in some cases at least, the separate bones of the coccyx to each other, by small cartilaginous disks like that connecting the sacrum with the last lumbar vertebra. They are further united by anterior and posterior common ligaments, the latter being much the thicker and more marked. In the adult female a synovial membrane is found between the sacrum and coccyx, and it is supposed that this is formed under the influence of the movements of the bones on each other.

Sacro-iliac Synchondrosis.-The opposing articular surfaces of the sacrum and ilium are each covered by cartilages, that of the sacrum being the thicker. These are firmly united, but, in the female, according to Mr. Wood,' they are always more or less separated by an intervening synovial membrane. Posterior to these cartilaginous convex surfaces there are strong interosseous ligaments, passing directly from bone to bone, filling up the interspace between them, and uniting them firmly. There are also accessory ligaments, such as the superior and anterior sacro-iliac, which are of secondary consequence. The posterior sacro-iliac ligaments, however, are of great obstetric importance. They are the very strong attachments which unite the rough surfaces on the posterior iliac tuberosities to the posterior and lateral surfaces of the sacrum. They pass obliquely downward from the former points, and suspend, as it were, the sacrum from them. According to Duncan, the sacrum has nothing to prevent its being depressed by the weight of the body but these ligaments, and it is mainly through them that the weight of the body is transmitted to the sacro-cotyloid beams and the heads of the femora.

The sacro-sciatic ligaments are instrumental in completing the canal of the pelvis. The greater sacro-sciatic ligament is attached by a broad base to the posterior inferior spine of the ilium, and to the posterior surfaces of the sacrum and coccyx. Its fibres unite into a thick cord, cross each other in an X-like manner, and again expand at their insertion into the tuberosity of the ischium. The lesser sacrosciatic ligament is also attached with the former to the back parts of the sacrum and coccyx, its fibres passing to their much narrower insertion at the spine of the ischium, and converting the sacro-sciatic notch into a complete foramen.

2

The obturator membrane is the fibrous aponeurosis that closes the large obturator foramen. Joulin supposes that along with the sacrosciatic ligaments, it may, by yielding somewhat to the pressure of the fetal head, tend to prevent the contusion to which the soft parts would be subjected if they were compressed between two entirely osseous surfaces.

Symphysis Pubis.-The junction of the pubic bones in front is effected by means of two oval plates of fibro-cartilage, attached to each articular surface by nipple-shaped projections, which fit into corresponding depressions in the bones. There is a greater separation between the bones in front than behind, where the numerous fibres of the cartilaginous plates intersect, and unite the bones firmly together.

1 Todd's Cyclopædia of Anatomy and Physiology, article " Pelvis," p. 123.

2 Traité d'Accouchements, p. 11.

At the upper and back part of the articulation there is an interspace between the cartilages, which is lined by a delicate membrane. In pregnancy this space often increases in size, so as to extend even to the front of the joint. The juncture is further strengthened by four ligaments, the anterior, the posterior, the superior, and the sub-pubic. Of these, the last is the largest, connecting together the pubic bones and forming the upper boundary of the pubic arch.

Section of pelvis and heads of thigh-bones, showing the suspensory action of the sacro-iliac ligaments. (After WOOD.)

Movements of Pelvic Joints.-The close apposition of the bones of the pelvis might not unreasonably lead to the supposition that no movement took place between its component parts; and this is the opinion which is even yet held by many anatomists. It is tolerably certain, however, that even in the unimpregnated condition there is a certain amount of mobility. Thus Zaglas has pointed out that in man there is a movement in an antero-posterior direction of the sacro-iliac joints which has the effect, in certain positions of the body, of causing the sacrum to project downward to the extent of about a line, thus narrowing the pelvic brim, tilting up the point of the bone, and thereby enlarging the outlet of the pelvis. This movement seems habitually brought into play in the act of straining during defecation.

During pregnancy in some of the lower animals there is a very marked movement of the pelvic articulations, which materially facilitates the process of parturition. This, in the case of the guinea-pig and cow, has been especially pointed out by Dr. Matthews Duncan.' In the former during labor the pelvic bones separate from each other

1 Monthly Journal of Medical Science, Sept. 1851.

2 Researches in Obstetrics, p. 19.

to the extent of an inch or more. In the latter the movements are different, for the symphysis pubis is fixed by bony ankylosis, and is immovable; but the sacro-iliac joints become swollen during pregnancy, and extensive movements in an antero-posterior direction take place in them, which materially enlarge the pelvic canal during labor.

It is extremely probable that similar movements take place in women, both in the symphysis pubis and in the sacro-iliac joints, although to a less marked extent. These are particularly well described by Dr. Duncan. They seem to consist chiefly in an elevation and depression of the symphysis pubis, either by the ilia moving on the sacrum, or by the sacrum itself undergoing a forward movement on an imaginary transverse axis passing through it, thus lessening the pelvic brim to the extent of one or even two lines, and increasing, at the same time, the diameter of the outlet, by tilting up the apex of the sacrum. These movements are only an exaggeration of those which Zaglas describes as occurring normally during defecation. The positions which the parturient woman instinctively assumes find an explanation in these observations. During the first stage of labor, when the head is passing through the brim, she sits, or stands, or walks about, and in these erect positions the symphysis pubis is depressed, and the brim of the pelvis enlarged to its utmost. As the head advances through the cavity of the pelvis, she can no longer maintain her erect position, and she lies down and bends her body forward, which has the effect of causing a nutatory motion of the sacrum, with corresponding tilting up of its apex, and an enlargement of the outlet.

nancy.

These movements during parturition are facilitated by the changes which are known to take place in the pelvic articulations during pregThe ligaments and cartilages become swollen and softened, and the synovial membranes existing between the articulating surfaces become greatly augmented in size and distended with fluid. These changes act by forcing the bones apart, as the swelling of a sponge placed between them might do after it had imbibed moisture. The reality of these alterations receives a clinical illustration from those cases, which are far from uncommon, in which these changes are carried to so extreme an extent that the power of progression is materially interfered with for a considerable time after delivery.

On looking at the pelvis as a whole, we are at once struck with its division into the true and false pelvis. The latter portion (all that is above the brim of the pelvis) is of comparatively little obstetric importance, except in giving attachments to the accessory muscles of parturition, and need not be further considered. The brim of the pelvis is a heart-shaped opening, bounded by the sacrum behind, the linea iliopectinea on either side, and the symphysis of the pubes in front. All below it forms the cavity, which is bounded by the hollow of the sacrum behind, by the inner surfaces of the innominate bones at the sides and in front, and by the posterior surface of the symphysis pubis. It is in this part of the pelvis that the changes in direction which the foetal head undergoes in labor are imparted to it. The lower border of this canal, or pelvic outlet (Fig. 4), is lozenge-shaped, is bounded by the ischiatic tuberosities on either side, the tip of the

coccyx behind, and the under surface of the pubic symphysis in front. Posteriorly to the tuberosities of the ischia the boundaries of the outlet are completed by the sacro-sciatic ligaments.

There is a very marked difference between the pelvis in the male and the female, and the peculiarities of the latter all tend to facilitate the process of parturition. In the female pelvis (Fig. 5) all the bones are lighter in structure, and have the points for muscular attachments much less developed. The iliac bones are more spread out, hence the greater breadth which is observed in the female figure, and the peculiar side-to-side movement which all females have in walking. The tuberosities of the ischia are lighter in structure and farther apart, and the rami of the pubes also converge at a much less acute angle. This greater breadth of the pubic arch gives one of the most easily appreciable points of contrast between the male and the female pelvis; the

pubic arch in the female forms an angle of from 90° to 100°, while in the male (Fig. 6) it averages from 70° to 75°. The obturator foramina are more triangular in shape.

The whole cavity of the female pelvis is wider and less funnelshaped than in the male, the symphysis pubis is not so deep, and, as the promontory of the sacrum does not project so much, the shape of the pelvic brim is more oval than in the male. These differences between the male and female pelvis are probably due to the presence

of the female genital organs in the true pelvis, the growth of which increases its development in width. In proof of this, Schroeder states that in women with congenitally defective internal organs, and in women who have had both ovaries removed early in life, the pelvis has always more or less of the masculine type.

Measurements of the Pelvis.-The measurements of the pelvis that are of most importance from an obstetric point of view are taken between various points directly opposite to each other, and are known as the diameters of the pelvis. Those of the true pelvis are the diameters which it is especially important to fix in our memories, and it is customary to describe three in works on obstetrics-the antero-posterior