THE four bones which compose the pelvis are united by five articulations: the junction of the coccyx with the sacrum; the sacro-vertebral junction; the symphysis pubis; and the sacro- iliac symphyses. Each of these articulations is a true arthrodia or movable joint; and each one is supplied with synovial membranes During gestation the secretion from these membranes is much more abundant. After the climacteric period, both the synovial secretion and the membranes themselves dry up and disappear. From the mode of their constriction, these joints admit of a slight sliding motion in every direction. Thus during gestation the force of all jars upon either or all these bones is expended upon these elastic, cartilaginous surfaces and synovial membranes; and the contents of the pelvis are in this manner preserved from serious injury. This will appear more clearly from a briefly particular description of each pelvic articulation.
In the COCCYGEAL JUNCTION, or connection of the coccyx with the sacrum, there are found fibro-cartilages which cover each articulating surface. Between these cartilages is placed the synovial capsule. Thus the sacro-coccygeal articulation admits of free motion, especially backwards, -by means of which, the inferior outlet of the pelvis may be enlarged at least one inch in its antero-posterior diameter,. This movement of the coccyx, so important in obstetric practice, is facilitated by the minor articulations of the small bones, which compose the coccyx itself. In some rare instances these fibro cartilages become completely ossified; as well at the junction of the coccyx with the sacrum, as at the points of union of the minor bones of the coccyx. In such cases the coccyx forms one continuous bone with the sacrum, greatly extending its curve, and constituting a formidable obstacle to the passage of the head through the inferior strait in parturition. But ordinarily the mobility of the coccyx as a whole, and of the parts which compose it, increases during pregnancy; and so affords an additional advantage in labor.
The anterior and posterior sacro-coccygeal ligament’s support this articulation in front and behind. While the inferior sacro- sciatic ligaments prevent lateral movement, by extending down extremity of the coccyx on each side.
In the SACRO- VERTEBRAL JUNCTION, or articulation of the sacrum with the last lumbar vertebra, is found the wedge-shaped fibro cartilage common to the inter-vertebral articulations. Between the oblique processes of the sacrum and those of the vertebra, appears the synovial membrane of this joint. Here the movements flexion, extension and rotation,-common to the true spinal articulations,- may all be accomplished, but to a limited extent only. The lumbar curve, which begins with the last dorsal vertebra, terminates at this sacro-vertebral junction or angle,- the pelvic curve commencing at the same angle and terminating at the extremity of the coccyx.
The lumbo-sacral and lumbo-iliac ligaments are those which more immediately connect the spine with the sacrum and pelvis. The obturator foramen is almost entirely covered and closed by the obturator membrane, which at once serves to complete the walls of the pelvis and give support to the parts.
The SYMPHYSIS PUBIS, or articulations of the pubic bones in front, is formed by means of two dense fibro-cartilaginous plates, which cover the oval and convex articulating surfaces of the ossa pubis. These cartilaginous plates are themselves connected by an intermediate, fibrous elastic tissue, especially where the opposing convex surfaces of the pubic bone recede from each other. While the articulating surface of each of the two cartilages, is composed of a little facet, smooth and furnished with a synovial membrane, which is more lubricated with synovia as the female approaches the period of labor. A considerable thickness of the inter-pubic ligament fills up the interval which exists between the other points of these articular surfaces. The articulation is arthrodial.
This articulation is strengthened by several ligaments which traverse it within and without, above and below. The superficial fibres of the anterior pubic ligament pass obliquely from one bone to the other, the deep fibres pass directly across the symphysis and blend with the interarticular fibro-cartilage beneath. The posterior pubic ligament, composed of some scattered fibres, unites the pubic bones posteriorly. The superior pubic ligament connects the pubis above. And the sub-pubic ligament connects them beneath, its inferior surface forming the upper boundary of the pubic arch; and its superior surface blending with the interarticular cartilage above and with the descending rami of the pubes on either side.
The strength and variety of these ligaments, which combine to strap the pubic bones together in every direction, and in the firmest possible manner, afford sufficient proof of the error of those obstetric writers, who have supposed the symphysis pubis was intended to separate for the sake of enlarging the diameter of the pelvis in parturition. Except in cases of serious deformity, the head is capable of being moulded (elongated) in its descent, so as to accommodate itself to the size and shape of the pelvis. But neither in such cases of serious deformity, nor in any other, does disarticulation of the pelvic or sacro-iliac symphysis enable the pelvis to adapt itself to the size of the descending head.
The SACRO-ILIAC SYMPHYSIS, of either side, is formed by the union of the superior, ear -shaped portion of the lateral border of the sacrum with the inferior, auricular portion of the posterior border of the ilium. Each of these articulating surfaces is covered with a layer of cartilage; the sacral layer being much thicker than the iliac. And both the sacral and iliac cartilaginous surfaces are covered by delicate synovial membrane, which, in the female,-especially in the pregnant state, -secretes a true synovial fluid. Thus, as in the symphysis pubis, so in the sacro-iliac symphysis, a limited sliding motion is provided for.
The sacro-iliac symphysis is strengthened by several important ligaments, which, without entirely preventing the sliding motion just mentioned, enable these joints to sustain in safety the whole weight of the body. The anterior sacro-iliac ligament, composed of numerous thin, bands, connects the anterior surface of the ilium and sacrum. The posterior sacro-iliac ligament occupies the depression between the sacrum and ilium, behind the edges of the cartilaginous surfaces, and forms the most powerful bond of union between these two bones. This ligament consists of numerous short interlacing fasciculi, which pass in every direction from the sacrum to the ilium, and which from their fibrous and elastic nature are admirably fitted to maintain the necessary stability of this important articulation. In this general name of posterior sacro-iliac ligament, are included also those which by some writers are described as superior and inferior sacro-iliac ligaments. In addition to these, the greater and lesser sacro-sciatic ligaments change the great sciatic notch of either side into a foramen; and at the same time serve to bind the ilium still more firmly to the sacrum, and contribute to the constriction of the walls of the pelvis.
THE PELVIS AS A WHOLE.
Having thus studied the separate bones which compose the pelvis, and examined the manner in which they are united and strengthened, and observed the nature of their articulations, we come to the consideration of the pelvis as a whole.
In general, the pelvis may be divided into an external and an internal surface, each of which needs to be studied by itself.
The EXTERNAL SURFACE of the pelvis presents four regions. The anterior region extends from the symphysis pubis, on the median line, to the edge of the cotyloid cavity on either side, and is principally covered by the obturator muscles. The posterior region is marked by the ridge formed by the spinous processes of the sacrum, the inferior opening of the vertebral canal, the union of the sacrum with the coccyx, and the posterior surface of the coccyx itself. Each lateral region is formed superiorly by the external iliac fossa, mediately by the cotyloid cavity, and inferiorly by the external surface of the tuberosity of the ischium.
On its INTERNAL SURFACE, the pelvis is divided into the upper and larger, and the lower and smaller pelvis excavation. The line of division extends on a level with the superior border of the symphysis pubis, along the linea pectinea and linen ilia to the promontory of the sacrum. Thus the superior pelvis is understood to include all the parts above and the inferior pelvis all those below the ilio-pectineal line. The superior pelvis forms the base of the abdomen; and during the later months of pregnancy supports the gravid uterus. The inferior pelvis is that which is principally concerned in parturition; and which the accoucheur should therefore endeavor fully to understand in all its parts and relations. Both the superior and the inferior pelvic divisions are again divisible into four-distinct regions.
