Look, Ma! Hands!
While the bones of the axial skeleton define the vertical axis of the body, the appendicular skeleton makes up the arms and legs, as well as the girdles that secure the limbs to the axial skeleton.
Pectoral Girdle
The bones that support the arms and compose the pectoral girdle are the scapulae and the clavicles. Also known as the collarbones, the clavicles are slender bones that attach and anchor the scapula to the manubrium of the sternum. They also act as a platform for the attachment of muscles from the arms, chest, and back.
Anatomy of a Word
ligaments
Ligaments are connective tissues that attach bone to bone and resist stretching. Tendons are made from the same material as ligaments, but connect muscle to bone.
The scapulae (shoulder blades) are the broad and flattened bones clearly visible on the back. Each scapula projects from the shoulder toward the spine. The broad flattened surfaces of the scapula are locations for the attachments of large muscles of the back, including the supra- and infraspinous muscles as well as the subscapular muscle. The scapula is somewhat triangular in shape, with the base toward the spine and the point toward the shoulder. The surface of the point is actually a concave depression that serves as a location for the ball of the upper arm bone (humerus) to form a freely movable joint with the scapula. This connection is stabilized by the ligaments and tendons of the shoulder joint.
Arms and Hands
Human arms are made up of only 3 long bones: the humerus, radius, and ulna. The humerus is the single bone of the upper arm. The radius and ulna make up the forearm. The head of the humerus fits into the depression of the scapula called the glenoid cavity (glenoid fossa). At the opposite end of the humerus, the humerus forms a joint (the elbow) with the joining (articular) surfaces of the radius and ulna. The capitulum, a small knob on the end of the humerus, and the trochlea, a pulley-shaped part of the humerus, fit together with the head of the radius and the trochlear notch of the ulna, respectively.
At the ends of the forearms, a collection of bones cluster together into the wrists, hands, and fingers. The 8 small, irregularly shaped bones adjacent to the radius and ulna are collectively called the carpals, and form a base upon which the longer bones of the hands and fingers are attached to the bones of the forearm. The first long bones that extend from the carpals to the fingers are the metacarpals; they are attached to the 3 remaining bones of each finger, the phalanges.
Pelvic Girdle
The pelvis is a composite structure composed of the sacrum (along the dorsal, or back-side aspect) and 2 hip bones, the coxal, or innominate, bones. Together these bones create a bottomless, basket-shaped girdle that supports the lower abdominal organs as well as provides an attachment point for the legs to the axial skeleton. This open-ended structure in females is wider than in males to facilitate the passage of the baby through the birth canal.
The 2 hip bones are joined to the sacrum along the back of the body at the sacroiliac joint. This is a relatively fixed joint, especially when compared to the joining of the 2 pelvic bones along the front of the body at the pubic symphysis. During pregnancy and at delivery, this joint allows for the expansion of the pelvic girdle and delivery of the baby through the birth canal.
Each pelvic bone has 3 regions. The broad blades on the back of the pelvis are the ilium portions of the hip bones. The ventral portion of the hip bone is divided into a superior (pubis) and inferior (ischium), around which is a large opening called the obturator foramen. At approximately the location where all 3 portions of the hip bone connect is a large depression where the ball of the femur (leg bone) connects. This depression is called the acetabulum and forms the socket of the hip ball and socket joint.
Legs
The legs have a layout similar to that of the arms. They are composed of a single upper leg bone (the femur), 2 lower leg bones (the tibia and fibula), and bones of the ankles and feet. The femurs support the full weight of the body. They join with the pelvic bone via the head of each femur fitting into the sockets of the acetabula in the hip.
Anatomy of a Word
condyle
A condyle is a rounded protuberance at the end of a bone, generally where it attaches to another bone and forms a joint.
At the knee, the medial and lateral condyles of the femur form the articular (joining) surface with the smooth surface of the tibia. This lower leg bone supports the full weight of the body whereas the fibula actually joins with the side of the tibia rather than directly contributing to the knee itself. The remaining bone of the knee, which is actually suspended within its own tendon, is the patella (knee cap).
At the ankle, the lower portion of the fibula (lateral malleolus) projects outward on the outer side. This rounded bump is clearly prominent on the surface. It is here, at the surface of both the tibia and fibula, where the ankle joint is formed between the bones of the lower leg and the tarsal bones of the foot. The most prominent of these tarsal bones is the calcaneus (heel bone). Similar to the wrists and hands, the first long slender bones attaching the toes (phalanges) to the foot are the metatarsals.
Joints
While joints are often thought of as the movable points of the human body, some are actually completely immovable. Thus, the definition of a joint in strict anatomical terms is the connection between two or more opposing bones, which may or may not allow for movement.
Ball and Socket Joints
This type of joint, as seen in the hip and the shoulder, is produced when the ball, or head, of a long bone inserts itself into the bowl-shaped depression of another bone, either of the pectoral girdle (glenoid cavity of the scapula) or the pelvic girdle (acetabulum of the pelvic bone). This type of joint allows for a wide range of rotational movement and gives the most flexibility of the body’s joints.
Synovial Joints
Synovial joints are also highly flexible, but only along a single plane. These are often called “hinge” joints, because they work like a door swinging open or shut. These joints (the elbow and knee) are covered in membranes that contain cells producing synovia, a lubricating fluid that can also cushion the surfaces of the bones that are in contact in much the same way that hydraulic fluid can withstand great pressures.
Fibrous Joints
These are immovable joints. In the adult human skull, they are seen as the suture lines between the bones of the skull. During the fetal period and into early childhood, the bones of the skull are not attached to one another in order to allow for the head to compress and pass through the birth canal. Later, these bones grow and fuse permanently with only the suture line as evidence that the bones were ever separated at all.