HUMAN BODY SYSTEMS Skeletal Muscular Circulatory Lymphatic Digestive Excretory Endocrine Reproductive Skin Respiratory

SKELETAL SYSTEM The skeleton is the framework of the human anatomy, supporting the body and protecting its internal organs. Two hundred and six bones compose the skeleton, about half of which are in the hands and feet. Most of the bones are connected to other bones at flexible joints, which lend the frame- work a high degree of flexibility. The skeletons of male and female bodies are the same, except female bones are usually lighter and thinner than male bones, and the female pelvis is shallower and wider than the male's. This latter difference makes childbirth easier.

The spinal, or vertebral, column is one of the primary support structures for the human skeleton. Made up of separate, pseudoseparate, and fused vertebrae, the spine features a great deal of articulation, allowing support and movement of the skull, flexion of the neck and back, anchor sites for the ribs (which enclose the abdominal cavity), and support and protection for the spinal cord. The spine is composed of seven cervical vertebrae, forming the neck, twelve thoracic vertebrae, which form the upper back, and five lumbar vertebrae, which form the lower back. Also part of the spinal column is the sacrum, a bone made of five fused vertebrae which anchors the spine to the pelvic girdle, and the coccyx, or tailbone, a semi-flexible series of four or less vertebrae which help to protect the lower alimentary tract.

The true ribs are the upper seven pairs in the ribcage. They are called true ribs because they articulate anteriorly to the sternum, distinguish- ing them from false ribs, which do not. The true ribs are also connected anteriorly to the spinal vertebrae. In both cases, the connection is cemented by costal cartilage. The ribcage is composed of twelve pairs of ribs which articulate with the vertebrae of the spinal column and the sternum (with some exceptions) to create the thoracic cavity.

The marrow cavity is the region within the bone which houses the marrow, responsible for generation of blood cells. The marrow in newborns and children is generally red, turning yellow in adults in all bones with the exceptions of upper ends of the humerus and femur and in many of the flat bones of the skull, vertebrae, ribs, sternum, and hip. The Haversian lamellae are layers of bone formed during the development of the bone which contain the Haversian canals. These canals house the blood vessels, lymph vessels, nerves, and marrow of the bone.

There are four types of movable joints in the human skeleton: pivot joints, ball-and-socket joints, hinge joints, and gliding joints. Each type of movable joint permits a different kind of movement. The fixed joints in the skull do not allow any motion. Click here for movie