The Lower Division of the Appendicular Skeleton

The bones of the lower division also fall into two groups, the pelvis and the lower extremity.
The Pelvic Bones The hip bone, or os coxae, begins its development as three separate bones that later fuse (Fig. 3-21). These individual bones are:
* The ilium, which forms the upper, flared portion. The iliac crest is the curved rim along the superior border of the ilium. It can be felt just below the waist. At either end of the crest are two bony projections. The most prominent of these is the anterior superior iliac spine, which is often used as a surface landmark in diagnosis and treatment.
* The ischium, which is the lowest and strongest part. The ischial spine at the posterior of the pelvic outlet is used as a point of reference during childbirth to indicate the progress of the presenting part (usually the baby’s head) down the birth canal. Just inferior to this spine is the large ischial tuberosity, which helps support the weight of the trunk when one sits down. One is sometimes aware of this projection of the ischium when sitting on a hard surface for a while.
* The pubis, which forms the anterior part. The joint formed by the union of the two hip bones anteriorly is called the pubic symphysis. This joint becomes more flexible late in pregnancy to allow for passage of the baby’s head during childbirth. Portions of all three pelvic bones contribute to the formation of the acetabulum, the deep socket that holds the head of the femur (thigh bone) to form the hip joint. The largest foramina in the entire body are found near the front of each hip bone, one on each side of the pubic symphysis. Each opening is partially covered by a membrane and is called an obturator foramen (see Fig. 3-21). The two ossa coxae join in forming the pelvis, a strong bony girdle completed by the sacrum and coccyx of the spine posteriorly. The pelvis supports the trunk and the organs in the lower abdomen, or pelvic cavity, including the urinary bladder, the internal reproductive organs, and parts of the intestine. The female pelvis is adapted for pregnancy and childbirth (Fig. 3-22). Some of the ways in which the female pelvis differs from that of the male are:
* It is lighter in weight;
* The ilia are wider and more flared;
* The pubic arch, the anterior angle between the pubic bones, is wider;
* The pelvic opening is wider and more rounded;
* The lower diameter, the pelvic outlet, is larger;
* The sacrum and coccyx are shorter and less curved.

Figure 3-21 The pelvic bones. (A) Anterior view. (B) Lateral view; shows joining of the three pelvic bones to form the acetabulum.

Figure 3-22 Comparison of male and female pelvis, anterior view. Note the broader angle of the pubic arch and the wider pelvic outlet in the female. Also, the ilia are wider and more flared; the sacrum and coccyx are shorter and less curved.

The Lower Extremity The lower extremity is also referred to as the lower limb, or simply the leg, although technically the leg is only the region between the knee and the ankle. The portion of the extremity between the hip and the knee is the thigh. The lower extremity consists of the following bones:
* The femur, the bone of the thigh, is the longest and strongest bone in the body. Proximally, it has a large ball-shaped head that joins the os coxae (Fig. 3-23). The large lateral projection near the head of the femur is the greater trochanter, used as a surface landmark. The lesser trochanter, a smaller elevation, is located on the medial side. On the posterior surface there is a long central ridge, the linea aspera, which is a point for attachment of hip muscles.
* The patella, or kneecap (see Fig. 3-1), is embedded in the tendon of the large anterior thigh muscle, the quadriceps femoris, where it crosses the knee joint. It is an example of a sesamoid bone, a type of bone that develops within a tendon or a joint capsule.
* There are two bones in the leg (Fig. 3-24). Medially (on the great toe side), the tibia, or shin bone, is the longer, weight-bearing bone. It has a sharp anterior crest that can be felt at the surface of the leg. Laterally, the slender fibula does not reach the knee joint; thus, it is not a weight-bearing bone. The medial malleolus is a downward projection at the distal end of the tibia; it forms the prominence on the inner aspect of the ankle. The lateral malleolus, at the distal end of the fibula, forms
the prominence on the outer aspect of the ankle. Most people think of these projections as their “ankle bones,” whereas, in truth, they are features of the tibia and fibula.
* The structure of the foot is similar to that of the hand. However, the foot supports the weight of the body, so it is stronger and less mobile than the hand. There are seven tarsal bones associated with the ankle and foot. These are named and illustrated in Figure 3-25. The largest of these is the calcaneus, or heel
bone.
* Five metatarsal bones form the framework of the instep, and the heads of these bones form the ball of the foot (see Fig. 3-25).
* The phalanges of the toes are counterparts of those in the fingers. There are three of these in each toe except for the great toe, which has only two.

Figure 3-23 The right femur (thigh bone).

Figure 3-24 Tibia and fibula of the right leg.

Figure 3-25 Bones of the right foot.

Figure 3-1 The skeleton. The axial skeleton is shown in yellow; the appendicular, in blue.