title="[Pg 16]"/> 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