Human Appendicular Skeleton

The appendicular skeleton is composed of the bones of the upper limbs (which function to grasp and manipulate objects) and the lower limbs (which permit locomotion). It also includes the pectoral girdle, or shoulder girdle, that attaches the upper limbs to the body, and the pelvic girdle that attaches the lower limbs to the body.

This figure shows the human skeleton. The left panel shows the anterior view, and the right panel shows the posterior view.

Figure 1: Appendicular Skeletons: The appendicular skeleton consists of the pectoral and pelvic girdles, the limb bones, and the bones of the hands and feet.

The Pectoral Girdle

The pectoral girdle bones provide the points of attachment of the upper limbs to the axial skeleton. The human pectoral girdle consists of the clavicle (or collarbone) in the anterior, and the scapula (or shoulder blades) in the posterior.

The clavicles are S-shaped bones that position the arms on the body. The clavicles lie horizontally across the front of the thorax (chest) just above the first rib. These bones are fairly fragile and are susceptible to fractures. For example, a fall with the arms outstretched causes the force to be transmitted to the clavicles, which can break if the force is excessive. The clavicle articulates with the sternum and the scapula.

The scapulae are flat, triangular bones that are located at the back of the pectoral girdle. They support the muscles crossing the shoulder joint. A ridge, called the spine, runs across the back of the scapula and can easily be felt through the skin (Figure 2). The spine of the scapula is a good example of a bony protrusion that facilitates a broad area of attachment for muscles to bone.

Illustration shows the pectoral girdles of the shoulder. Each girdle consists of a long, thin clavicle that runs from the sternum to the arm and a flat, triangular scapula that extends down from the clavicle. Viewed from the back, the upper part of the scapula has a prominent protrusion, called a spine.

Figure 2: (a) The pectoral girdle in primates consists of the clavicles and scapulae. (b) The posterior view reveals the spine of the scapula to which muscle attaches.

The Upper Limb

Illustration shows a skeletal human arm. The humerus is the bone of the upper arm. The radius is the thick bone in the forearm, and the ulna is the thin bone. The carpals are the bones of the wrist, the metacarpals are bones of the hand, and phalanges are bones of the fingers.

Figure 3: Bones of The Upper Limb.

The upper limb contains 30 bones in three regions: the arm (shoulder to elbow), the forearm (ulna and radius), and the wrist and hand (Figure 3).

An articulation is any place at which two bones are joined. The humerusis the largest and longest bone of the upper limb and the only bone of the arm. It articulates with the scapula at the shoulder and with the forearm at the elbow. The forearm extends from the elbow to the wrist and consists of two bones: the ulna and the radius. The radius is located along the lateral (thumb) side of the forearm and articulates with the humerus at the elbow. The ulna is located on the medial aspect (pinky-finger side) of the forearm. It is longer than the radius. The ulna articulates with the humerus at the elbow. The radius and ulna also articulate with the carpal bones and with each other, which in vertebrates enables a variable degree of rotation of the carpus with respect to the long axis of the limb. The hand includes the eight bones of the carpus (wrist), the five bones of the metacarpus(palm), and the 14 bones of the phalanges(digits). Each digit consists of three phalanges, except for the thumb, when present, which has only two.

The Pelvic Girdle

This figure shows the structure of the female pelvic girdle on the left and the male pelvic girdle on the right.

Figure 4: Overview of Differences between the Female and Male Pelvis

Illustration compares male and female pelvic bones. In both males and females, a wide, rounded bone called the ilium attaches to each side of the spine. The ilium curves toward the front, where it narrows into the ischium. A loop-shaped bone extends down from the place where the ilium meets the ischium, and connects back to the ilium in the front center of the body.

The pelvic girdle attaches to the lower limbs of the axial skeleton. Because it is responsible for bearing the weight of the body and for locomotion, the pelvic girdle is securely attached to the axial skeleton by strong ligaments. It also has deep sockets with robust ligaments to securely attach the femur to the body. The pelvic girdle is further strengthened by two large hip bones. In adults, the hip bones, or coxal bones, are formed by the fusion of three pairs of bones: the ilium, ischium, and pubis. The pelvis joins together in the anterior of the body at a joint called the pubic symphysis and with the bones of the sacrum at the posterior of the body.

The female pelvis is slightly different from the male pelvis. Over generations of evolution, females with a wider pubic angle and larger diameter pelvic canal reproduced more successfully. Therefore, their offspring also had pelvic anatomy that enabled successful childbirth.

 
Overview of Differences between the Female and Male Pelvis
Female pelvis Male pelvis
Pelvic weight Bones of the pelvis are lighter and thinner Bones of the pelvis are thicker and heavier
Pelvic inlet shape Pelvic inlet has a round or oval shape Pelvic inlet is heart-shaped
Lesser pelvic cavity shape Lesser pelvic cavity is shorter and wider Lesser pelvic cavity is longer and narrower
Subpubic angle Subpubic angle is greater than 80 degrees Subpubic angle is less than 70 degrees
Pelvic outlet shape Pelvic outlet is rounded and larger Pelvic outlet is smaller

 The Lower Limb

Illustration shows a leg. The bone of the upper leg is the femur. The tibia is the thicker, front bone of the lower leg, and the fibula is the rear bone. The tarsals are the bones of the ankle. The metatarsals are the bones of the foot, and the phalanges are the bones of the toes.

Figure 5. The lower limb consists of the thigh (femur), kneecap (patella), leg (tibia and fibula), ankle (tarsals), and foot (metatarsals and phalanges) bones.

The lower limb consists of the thigh, the leg, and the foot. The bones of the lower limb are the femur (thigh bone), patella (kneecap), tibia and fibula (bones of the leg), tarsals (bones of the ankle), and metatarsals and phalanges (bones of the foot) (Figure 5). The bones of the lower limbs are thicker and stronger than the bones of the upper limbs because of the need to support the entire weight of the body and the resulting forces from locomotion. In addition to evolutionary fitness, the bones of an individual will respond to forces exerted upon them.

The femur, or thighbone, is the longest, heaviest, and strongest bone in the body. The femur and pelvis form the hip joint at the proximal end. At the distal end, the femur, tibia, and patella form the knee joint. The patella, or kneecap, is a triangular bone that lies anterior to the knee joint. The patella is embedded in the tendon of the femoral extensors (quadriceps). It improves knee extension by reducing friction. The tibia, or shinbone, is a large bone of the leg that is located directly below the knee. The tibia articulates with the femur at its proximal end, with the fibula and the tarsal bones at its distal end. It is the second largest bone in the human body and is responsible for transmitting the weight of the body from the femur to the foot. The fibula, or calf bone, parallels and articulates with the tibia. It does not articulate with the femur and does not bear weight. The fibula acts as a site for muscle attachment and forms the lateral part of the ankle joint.

The tarsals are the seven bones of the ankle. The ankle transmits the weight of the body from the tibia and the fibula to the foot.

The metatarsals are the five bones of the foot. The phalanges are the 14 bones of the toes. Each toe consists of three phalanges, except for the big toe that has only two.