© 2000 PATTS
The skeletal system (bones and joints), working interdependently with the skeletal muscle system (voluntary or striated muscles), provides basic functions that are essential to life:
A living bone consists of three layers, all honeycombed with nerves and blood vessels: 1) the periosteum, or outside skin of the bone; 2) the hard compact bone, supporting the weight of the body; and 3) spongy bone (bone marrow). Spongy bone occurs at the ends of long bones and is less dense than compact bone. The spongy bone of the femur, humerus, and sternum contains red marrow, producing red blood cells (which carry oxygen), white blood cells (which fight infection), or platelets (that help stop bleeding). Yellow marrow, at the center, is used to store fats.
A specialized form of connective tissue, bone consists of both organic components (e.g. collagen) and inorganic minerals (calcium, phosphorus, magnesium, potassium, and sodium). The minerals calcium and phosphorus give bone its hardness, strength, and rigidity to resist compressive forces. The collagen fibers impart flexibility. Magnesium, sodium, potassium, and other trace elements act as "mortar" bonding the calcium and phosphorous. The bone cells themselves are embedded in a mineralized calcium "matrix" and collagen fibers.
Bone continuously remakes itself: New bone is produced and old bone is removed. Osteoblasts, the cells responsible for making bone, maintain the balance of calcium in the blood and bone. When this balance is disrupted, as in osteoporosis, the removal of bone exceeds its production, making bone thin and brittle, thus more easily fractured. The intestines, vitamin D, the kidney, parathyroid gland, and sex and adrenal hormones also play important roles in bone/calcium balance. In long bone, illustrated above, growth occurs at the diaphysis side (shaft) of the epiphyseal plate, thus increasing the length of the shaft. Long bone growth stops when the hyaline cartilage stops reproducing itself and fully converts to bone.
or articulation, is a union of two or more bones.
attach bone to bone, stabilizing and strengthening joints and determining
the range of motion. Cartilage, a gel-like substance high in proteoglycans,
provides protective cushioning. There are three types of cartilage: 1)
(found in intervertebral discs), 2) elastic
cartilage (found in the external ear and epiglottis), and 3) hyaline
cartilage. Hyaline (or articular) cartilage is
the most important cartilage: It serves as the "original" skeleton in the
embryo from which bones develop; it spurs growth of long bones; and it
lines and protects joints.
Joints are classifiied into three groups: 1) immovable (fibrous) joints, e.g. skull bones; 2) slightly movable (cartilagenous) joints, e.g. intervertebral discs; and 3) freely movable (synovial) joints, e.g. limb joints. Synovial joints permit the greatest degree of flexibility and have the ends of bones covered with a connective tissue (synovial membrane) filled with joint (synovial) fluid.
A typical synovial joint, seen at right, has four main featues:
The average human adult skeleton consists of 206 bones, attached to the muscles by tendons. Babies are born with 270 soft bones - about 64 more than an adult. These will fuse together by the age of twenty or twenty-five into the 206 hard, permanent bones.
The skeleton has two main parts: the axial skeleton and the appendicular skeleton. The axial skeleton consists of the skull, the spine, the ribs and the sternum (breastbone) and includes 80 bones. The appendicular skeleton, consisting of 126 bones, includes two limb girdles (the shoulders and pelvis) and their attached limb bones.
Axial Skeleton (80 bones)
The vertebral body and the neural arch encircle the vertebral foramen. Stacked one on top of the other, these foramina form the vertebral canal, where the spinal cord resides.
Several structures strengthen the attachments between vertebrae: 1) anterior longitudinal ligaments in front of vertebral bodies and discs; and 2) posterior longitudinal ligaments behind bodies and discs; 3) the compact bone of the disc itself; 4) the interlocking hyaline cartilage surfaces of the neural arch joints; and 5) the ligaments attaching spinous processes to transverse processes.The intervertebral discs provide shock absorption.
The orientation of the neural arch joints determines allowable motions: 1) the cervical spine () to rotate, flex forward, flex sideways, and extend backward; 2) the thoracic spine () to rotate; and 3) the lumbar spine () to flex forward, flex sideways, and extend backward. The sacrum () has a dual character, being part of both the vertebral column and pelvis. As such, it transmits the upper body weight to the lower exterminites.
Appendicular skeleton (126 bones, 64 in the shoulders and upper limbs and 62 in the pelvis and lower limbs)
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© PATTS, Last update: October 2000, Maintained by Lane Price Rose