Next we’ll take a further look at the bones of the pelvis. For a start, we need to understand something about the word “pelvis”. The pelvis is commonly used as the short name for two different things: the bony pelvis, and the pelvic cavity.
This is the bony pelvis, consisting of the sacrum, and the two hip bones. This is the pelvic cavity: it’s the deep and narrow space that’s enclosed by the sacrum, the lower parts of the hip bones, and by ligaments and muscles that we’ll see in the next section. The pelvic cavity is continuous with the abdominal cavity here, at the pelvic brim, which we’ll return to in a minute.
To understand the abdomen, we need to understand the upper and anterior parts of the bony pelvis. We took a good look at the sacrum in the first section of this tape. The only parts of it that we’ll mention here are the most anterior part, the promontory, and this broad part, the ala or wing of the sacrum
Now we need to take a look at some important features of the hip bone. The hip bone is also known as the innominate bone. It develops by the fusion of three originally separate bones, the massive ischium below and behind, the more lightly constructed pubis below and in front, and the broad ilium above. We’ll look at the ilium first
The thick lower part of the ilium is the body. The broad expanse of bone that fans out above the body is the ala, or wing of the ilium. The concavity on the inner surface of the ala is known as the iliac fossa. This roughened area, the auricular surface, articulates with the sacrum, forming the sacro-iliac joint
The broad, roughened edge of the ala of the ilium is the iliac crest. The iliac crest ends behind at the posterior superior iliac spine. It ends in front at this important projection, the anterior superior iliac spine. The iliac crest lies close to the surface all the way along its length. The anterior superior iliac spine is here.
Next we’ll look at the pubis. This is the superior pubic ramus. This is the body of the pubis; and this is the ischio-pubic ramus. This thick ridge is the pubic crest, which ends at this prominence, the pubic tubercle.
Lateral to the tubercle, the superior ramus of the pubis has a sharp upper border,
called the pecten.
The two hip bones are held together in front by a cartilaginous joint, the pubic symphysis. The front of the pubic symphysis is here.
The pelvic brim, the narrowing which forms the open end of the pelvic cavity,
is made up by the pubic symphysis in front; by the promontory of the sacrum behind; and on each side by the ala of the sacrum, this shoulder on the body of the ilium called the arcuate line, and by the superior ramus of the pubis.
The parts of the bony pelvis that lie below the pelvic brim, the ischium, and the ischio-pubic ramus, don’t concern us at present. We’ll look at them in the next section.
As we’ve looked at the features of the bony pelvis, we’ve been seeing them in the position they’re in, when we’re standing upright. It’s useful to keep in mind certain basic planes and angles. In the normal standing position, the anterior superior iliac spines, and the front of the pubic symphysis, are in the same vertical plane when seen from the side.
The plane of the pelvic brim is tilted at about 60° to the horizontal. The pelvic surface of the sacrum slopes at 30° to the horizontal. The pubic symphysis is tilted at almost the same angle.
It’s also useful to keep in mind the distance between the costal margin above, and the pelvis below, and the way that distance changes from the front, to the side. In front, it’s a long way from the xiphoid process, to the pubic symphysis. But at the side, the costal margin, and the iliac crest, are only this far apart.