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 foetus 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_.