Labs 8, 9, & 10- Inguinal Region, Gonads, Perineum, and Pelvis

Suggested reading:
Ch. 4: pp. 260-261, 282-291, clinical case 5

Pre-lab on osteology and myology of the pelvis with a focus on the interior of the pelvis.

This pre-lab covers the osteology of the innominate bones and the way that these bones, along with the sacrum, form the pelvis.  The hip joint and the muscles moving the hip will be largely covered in later lectures. 

This pre-lab material is useful for preparation for the GA labs on the inguinal region, perineum, and pelvic contents.

A. The parts of the innominate bones:

What we think of as the “hip” bone is called the 'innominate bone' (figure at left).

There are two innominate bones (also called os coxae) that articulate with the sacrum dorsally and with each other ventrally to form the pelvis.

The innominate is actually a three-part bone that fuses during development.  The ilium (pink on left) is the superior half of the bone.  The pubis (light green on left) represents the anterior lower half and comes together in a symphysis with a cartilaginous pad.  Finally the ischium (beige on left) forms the posterior lower half. 


B. Key landmarks of the innominate bones:

The three pelvic bones converge and fuse at the acetabulum (“hip joint”) (center of red lines in image at left). There are important landmarks to note including:

  1. The iliac blades

  2. The greater sciatic notch which is formed by the expansion of the iliac blade (muscles, nerves, and vessels pass through here).

  3. The ischial spine is the attachement site for the sacrospinous ligament.

  4. The ischial tuberosity is the attachment site for sacrotuberous ligament.

(note that both ischial spine and tuberosity can be palpated on patients)

  1. The acetabulum is the “hip socket”.

  2. The pubic symphysis; the right and left of which articulate with a cartilaginous pad.

  3. The articular surface of the ilium which connects to the sacrum in a true synovial joint.

  4. The anterior superior iliac spine which has attachments to the inguinal ligament and Sartorius muscle.

  5. The anterior inferior iliac spine near the hip socket that attaches the iliofemoral ligament and the Rectus femoris muscle.

  6. The obturator foramen which is covered with a membrane of the same name that is an origin site for obturator muscles.  Obturator nerves and vessels pass through the foramen.



C. The ligaments of the pelvis:

The two inominate bones attach ventrally at the pubic symphysis and dorsally by articulating with the sacrum in joints with limited mobility.

The circle running from the pubic symphysis forms a pelvic rim, inferior to which is the true pelvis which contains the pelvic visera.


The articulations of the pelvis at the sacrum is heavily reinforced by notable and dissectable ligaments.  These include:

  1. Sacro-iliac ligaments around the ala of the sacrum.
  2. The lumbosacral ligaments that connect sacrum to lumbar vertebrae.
  3. The iliolumbar ligaments.

These three ligaments help prevent downward and upward motion of the upper half of the sacrum. Please remember that the sacrum is angled relative to the lumbar vertebrae and so the top half will be pushed downward and forward by body weight whereas the bottom half will rotate backward and upward).


There are two huge ligaments that help prevent the backward and upward rotation of the sacrum.  These ligaments are so huge that they will feel like thick rods and have very little flexibility.  They are:

  1. The sacrotuberous ligament
  2. The sacrospinous ligament

The ligaments are named for where they attach.  Both originate from the sacrum and one goes to the ischial tuberosity and the other to the ischial spine.  They cross each other and divide the space into the greater and lesser sciatic foramina.

Later you will learn that important muscles, nerves, and arteries pass through these foramina.


D. Muscles of the pelvis that you will encounter in these labs:

We will review muscles of the hip in later labs.  But there are some muscles you will see in the next two labs that are worth mentioning.  First you will see a muscle covering the obturator foramen. This is the Obturator internus that eventually sends its tendon out to the hip.  The other is the Piriformis that exits through the sciatic foramen and attaches to the femur.


Finally you will also see the pelvic floor muscles that are discussed in more detail in the labs on the perineum and on pelvic contents.




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