What forms the coxal bone?

In this anatomy lesson, I’m going to cover the pelvic girdle, which consists of the two hip bones. The hip bones are referred to by different names, such as os coxae or coxal bones, innominate bones, or the pelvic bones.

The pelvic girdle is part of the appendicular skeleton, and it not only protects the organs in your pelvic region, but it also attaches the lower limbs of the appendicular skeleton to the sacrum of the axial skeleton.

Are the Hipbones Classified as Irregular or Flat Bones?

Most anatomists today classify the hip bones as irregular bones, though some sources have classified them as flat bones in the past, so be aware of that.

Hip Bone Anatomy (Pelvic Girdle)

Each hip bone is divided into three main regions: The ilium, ischium, and pubis. These three regions begin as separate bones in youth, but they later fuse together to form one solid hip bone. The image below is colored to help you visualize the different areas, which helps in identifying landmarks.

These three hip regions meet in and around the acetabulum, which is the deep socket of the hip bone. The acetabulum is easily recognized in a lab exam, and it receives the head of the femur to form the acetabulofemoral joint, a ball-and-socket synovial joint.

The word acetabulum sounds a lot like “acid tablets,” and the word literally means vinegar cup. So I remember that this is the little cup of vinegar that I need to swallow my acid tablets.

Now let’s talk about each major region of the hip bone, starting with the ilium.

Ilium Anatomy Landmarks and Features

The ilium makes up the superior (or upper) region of the hip bone, and it is named after a Latin word that means “flank” or “entrails,” because that’s what it supports.

The ilium can be subdivided into two portions:

1. an inferior region (near the acetabulum) called the “body”
2. a superior, fanned out portion called the “wing,” or ala

• Iliac crest – if you feel your sides with your hand, you’ll probably feel the top of that wing-like portion of the ilium, called the iliac crest. This allows for the attachment of various muscles, and an easy way to remember this is to think back to your nine abdominal regions. These two lower regions are called the left and right iliac regions, so that can help you remember the iliac crest region.
• Tubercle of iliac crest – The iliac crest thickens around a small rounded projection called the tubercle of the iliac crest. When we examine the nine abdominal regions, you’ll notice that the inferior horizontal plane is called the intertubercular plane, because this plane passes through the tubercle on each bone of the iliac crest.
• Anterior superior iliac spine – On the anterior (or front) side of the ilium, the ilium has two points (spines). The superior (or top) spine is called the anterior superior iliac spine, and it allows for the attachment of the inguinal ligament, as well as the sartorius muscle.
• Anterior Inferior iliac spine – The anterior inferior iliac spine allows for the attachment of the the rectus femoris muscle, as well as the iliofemoral ligament of the hip joint.
• Posterior superior iliac spine – On the posterior (or back) side of the ilium, we also have two spines. The posterior superior (top) iliac spine allows for the attachment of part of the posterior sacroiliac ligaments, as well as the multifidus.
• Posterior inferior iliac spine – The posterior inferior (lower) iliac spine is just below the superior iliac spine.
• Greater sciatic notch – Just under the posterior inferior iliac spine, you’ll notice a huge notched area, which is called the greater sciatic notch. This notch allows for the passage of the sciatic nerve, and it will help you identify the posterior region of the hip bone.
• Gluteal surface – From the lateral (side) view of the ilium, you can see three distinct lines on the ala (wing) that are named after their relative directional terms (posterior, anterior, and inferior gluteal lines). These lines provide attachment for gluteal muscles.
• Iliac fossa – When you flip the hip bone around and look at the medial (inner) view, you’ll notice the large depression at the anterior region, called the iliac fossa. This is where the iliacus muscle attaches.
• Auricular surface – The auricular surface is on the posterior region of the ilium, and this articulates with the sacrum of the vertebral column to form the sacroiliac joint.
• Arcuate line – Finally, we have a prominent line on the medial side that extends from the auricular surface down to the pubis, called the arcuate line. This line serves as a landmark that separates the body of the ilium from the ala (wing).

Ischium Anatomy Landmarks and Features

Now let’s talk about the ischium of the hip bone (coxal bone), which is the lower rear portion (or posterior inferior region) of the hip bone.

Like the ilium, the ischium can be divided into two main regions:

1. The ischial body, which makes up the superior region of the ischium and forms approximately two-fifths of the acetabulum
2. The ischial ramus, which is a branch-like structure that makes up the inferior portion of the ischium and connects to the inferior ramus of the pubis

• Ischial spine – On the posterior region, you’ll notice a projection called the ischial spine. This allows for the attachment of various muscles and ligaments: gemellus superior, coccygeus, levator ani, pelvic fascia, and sacrospinous ligament.
• Lesser sciatic notch – Below the ischial spine is the lesser sciatic notch, which allows the passage of nerves and vessels.
• Ischial tuberosity – On the lateral side of the ischium is the ischial tuberosity, a roughened bump that allows for the attachment of the sacrotuberous ligament, as well as several muscles: adductor magnus, the biceps femoris, and the semitendinosus.

Pubis Anatomy Landmarks and Features

Finally, we have the last major section of the hip bone, the pubis. The word pubis means “sexually mature,” and by thinking of your pubic region and private parts, you can remember that this is the area of the hip bone that is the anterior (front) portion. You can usually feel your pubic bone if you palpate in that region.

Like the ischium and ilium, the pubis also has a body, but it has two rami coming off the body, which allow for the attachment of various muscles and ligaments.

• Superior ramus – The superior ramus is the upper portion of the pubis that branches out. There is a ridge on superior border of this ramus called the pectineal line, which allows for the attachment of various ligaments and muscles, and it forms part of the pelvic brim.  Some of the structures that attach to this include the lacunar ligament, conjoint tendon, pectineal ligament, pectineus muscle, and psoas minor muscle.
• Inferior ramus – The inferior ramus is the lower portion of the pubis that branches out.
• Obturator foramen – These two rami of the pubis join the ischium, forming a large hole in the hip bone called the obturator foramen. The word foramen refers to a hole in a bone, and this is the largest “bone hole” in the human body. This huge hole allows for the passage of the obturator artery, vein, and nerve. The obturator muscles also attach near this region. I had an anatomy professor in college who told us that to remember the obturator muscle, we can imagine a guy running his hand up a girl’s leg, thinking that he’s some kind of smooth obturator (like the song).
• Pubic crest – On the anterior (front) side of the pubis body, you’ll find a rim of bone called the pubic crest, which allows for the attachment of the inguinal falx, as well as the abdominal external oblique and pyramidalis muscles.
• Pubic tubercle – There is a small bump where the pubic crest (laterally) joins with the superior pubic ramus (medially), called the pubic tubercle, to which the inguinal ligament attaches.
• Pubic symphysis (or symphysis pubis) – The two pubic bones are joined together by a cartilaginous joint called the pubic symphysis, which is a nonsynovial amphiarthrodial joint.
• Pubic arch – Under this pubic symphysis, you’ll notice that the bones form an arch or notch, which resembles and upside down “V.” This is called the pubic arch, and it has a much smaller angle in men than in women (see male vs female pelvis). I’m going to compare and contrast the male and female pelvis in a future video, and also talk about the true and false pelves (so stay tuned for that!).

Free Quiz and More Anatomy Videos

Take a free hip bones anatomy quiz to test your knowledge. In addition, you might want to watch our anatomy and physiology lectures on YouTube, or check our anatomy and physiology notes.