THE PELVIS IN GENERAL.
Considered as a whole, the pelvis, formed by the union of the sacrum, the coccyx and the ossa innominata, and of the union of the sacrum with the vertebral column, has the shape of a bony canal, larger above than below, slightly flattened from in front, backwards.
Exterior Surface.—On the external surface of the bones which form the pelvis are found the parts already described. These have no impor tance from the obstetrician's point of view.
Internal Surface.—It is not so with the internal surface. This de serves, and calls for, our whole attention.
Smooth and regular, it is divided into two parts; one, the superior, large and wide, the greater pelvis; the other the inferior, narrower, the lesser pelvis. The larger pelvis is separated from the lesser by the su perior strait, the lesser pelvis terminates in the inferior strait.
The Greater Pelvis.
It is formed by all that part of the internal surface comprised between the iliac crests, above, and the point whera it narrows below, i.e., by the line which forms the superior strait (Fig. 11).
In front we find a large notch; on the sides the iliac-fossie, behind a protuberance formed by the sacro-vertebral angle, or promontory, and the body of the last lumbar vertebra. On the sides of this promontory is a deep groove, formed on the inside by the body of the same vertebra, the anterior face of the transverse process of that vertebra, and the ilio lumbar ligament, on the outside, by the posterior surface of the internal iliac-fossa, and below by the quadrilateral surface which forms the wing of the sacrum.
The superior edge or circumference of the greater pelvis, which is broken at the level of the notch of the greater pelvis, is formed, pos teriorly, by the superior face of the last lumbar vertebra, the transverse process of that vertebra, the ilio-lumbar ligament and the anterior four fifths of the iliac crest.
In the living subject the notch is closed by the abdominal walls, which are attached to the iliac crests, and to the anterior edge of the ossa inno minata.
Size.—From one anterior and superior iliac spine to the other the greater pelvis measures on the average from nine to ten inches.
From one iliac crest to the other, at the widest point, about 10i inches. A line from the middle of the iliac crest to the middle of the linea in nominate measures about 3.2 inches.
The Lesser Pelvis.
This is that portion of the pelvic canal which is situated below the greater pelvis. It is shaped like a canal slightly narrowed at its two ends. It is composed of:— The middle part or cavity.
The superior opening of that cavity, the superior or abdominal strait.
The inferior opening, the inferior or perineal strait.
Pelvic Cavity.
the pelvic cavity is limited above by the superior strait, and below by the inferior strait. It has four walls, one anterior, one posterior and two lateral.
Anterior Wall.—This is included between two imaginary lines running from the ilio-pectineal eminence to the internal face of the tuberosity of the ischium, and follows the external edge of the obturator canal. Here we find the pubic symphysis, and the prominence which we have already noticed in connection with the inter-pubic cartilage, the internal face of the body of the pubes, the obturator or sub-pubic canal, the internal face of the ischio-pubic ramus and of the tuberosity of the ischium (Dubois and Pajot).
Posterior Wall.—This is included between two lines which run from the anterior and superior part of the sacro-iliac symphysis to the inferior edge of the great sciatic ligament, close to its insertion on the sacrum and coccyx. It is triangular, and concave like the sacrum which forms its greater part.
Here are found: the anterior face of the sacrum and coccyx, the verte bral foss of the sacrum, the transverse lines which separate them and the anterior sacral foramina. In some cases the line which separates the first two pieces of the sacrum is so pronounced, as to be taken for the sacro - vertebral angle.