skeletal

Axial Skeleton. The axial skeleton includes the skull, the spine, and the ribs and sternum. The most important of these is the spine (called also the spinal or vertebral column), consisting of 26 separate bones. Twenty-four vertebrae have holes through them, which are lined up vertically to form a hollow tube called the spinal canal; the spinal cord runs through this canal and is protected by it.



The seven topmost vertebrae, in the neck, are the cervical vertebrae; they support the skull, which encloses and protects the brain and provides protection for the eyes, inner ears, and nasal passages. The skull includes the cranium, the facial bones, and the auditory ossicles. Of the 28 bones of the skull, only one, the mandible, is movable.

Below the cervical vertebrae are 12 thoracic vertebrae; attached to them are 12 pairs of ribs, one pair to a vertebra. The ribs curve around to the front of the body, where most attach directly to the sternum or are indirectly attached to it by means of cartilage. The two bottom pairs of ribs are unattached in front and are called floating ribs. Together, the thoracic vertebrae, the ribs, and the sternum form a bony basket called the thoracic (or rib) cage, which prevents the chest wall from collapsing and protects the heart and the lungs. The remaining bones of the spine include five lumbar vertebrae, which support the small of the back, and the sacrum and coccyx. The axial skeleton also includes a single bone in the neck, the hyoid bone, to which muscles of the mouth are attached. This is the only bone of the body that does not join with another bone.

Appendicular Skeleton. The appendicular skeleton includes the shoulder girdle, bones of the upper limb, pelvic girdle, and bones of the lower limb. The shoulder girdle, from which the arms hang, consists of two clavicles and two scapulae; the scapulae are joined to the sternum.



The upper limb has three long bones. The uppermost bone is the humerus, whose upper (proximal) end fits into a socket in the shoulder girdle; its lower (distal) end is connected at the elbow to the ulna and radius, the two long bones of the forearm. Eight small bones, the carpals, compose the wrist. Five metacarpals form the palm of the hand, and the finger bones are made up of 14 phalanges in each hand.

At the lower end of the spine is the pelvic girdle; it, along with the last two bones of the spinal column (the sacrum and coccyx), forms the pelvis. This part of the skeleton encircles and protects the internal organs of the genitourinary system. In each side of the pelvis is the acetabulum, a socket into which a femur fits.

The bones of the lower limb are similar in construction to those of the upper limb but are heavier and stronger. The femur (thigh bone), which is the longest bone in the body, extends from the pelvis to the knee. The tibia and the fibula are long bones that extend from the knee to the ankle. On the knee is another single bone, the patella or kneecap. In each leg there are seven ankle bones, or tarsals; five foot bones, or metatarsals; and 14 toe bones, or phalanges.