The skeletal system
The skeletal system is the body's framework of bones, cartilage, joints and connective tissue. It supports the body, protects delicate structures, stores compounds and acts as a lever system for movement. A long bone has a compact shaft and spongy ends that hold marrow, bones are classified by their shape and identified by their markings, and joints are grouped by how much movement they allow. Bone grows during the growth period of life and is constantly remodelled in both children and adults, and cartilage resembles bone but differs from it.
Parts and functions of the skeleton
The skeletal system comprises bones, cartilage, joints or articulations, and connective tissue, including ligaments, which connect bone to bone, and tendons. A ligament is a tough, fibrous band connecting bones or visceral organs. The skeletal system forms a supporting framework, protects delicate structures such as the brain, stores compounds such as calcium salts, and acts as a lever system for movement. The body contains 206 bones and associated cartilages. The skeleton is divided into axial and appendicular portions. The axial portion includes the skull, spine and thorax, or ribs. The appendicular portion includes the bones in the arm and forearm, leg and thigh, wrist and hand, and ankle and foot, and their girdles, or attachments, to the skeleton.
The structure of a long bone
In a typical long bone, the diaphysis is the elongated cylindrical shaft between the epiphyses, a hollow tube of hard compact bone enclosing the medullary, or marrow, cavity. The medullary cavity is the hollow area inside the diaphysis, and it contains yellow fatty marrow. The epiphyses are the ends of the long bone, usually articular and wider than the shaft, and each is a center for ossification of the long bone. They are broad to provide stability to the joint. Red marrow fills the small spaces in the spongy, or cancellous, bone of the epiphyses. Cancellous bone has a reticular, lattice-like or spongy structure and contains marrow. Because red marrow has a role in hematopoiesis, the bones of infants and children have more red marrow than the bones of adults. As humans age, much of the red marrow transforms into yellow marrow, an inactive fatty tissue. The bone is covered by a strong fibrous membrane, the periosteum, except at the joint surface, where it is covered by articular cartilage, or gristle. A similar membrane, the endosteum, lines the interior cavity of the bone.
Compact bone and bone cells
Compact bone is composed of individual units, like tree rings, called Haversian canals. A Haversian canal is one of the interconnecting channels in compact bone. A blood vessel runs through the middle of the canal, and small branches, canaliculi, ensure that the living cells of bone receive nutrients and oxygen. A canaliculus is an extremely narrow tubular passage, a connection between cells in compact bone. Bone cells, the osteocytes, are alive and are embedded in lacunae within a hard, calcified, nonliving matrix. A lacuna is a small pit or hollow cavity in compact bone that contains a bone cell. The canaliculi connect the lacunae with one another.
Bone shapes and markings
Bones may be categorized by shape as long, short, flat, irregular, sesamoid or sutural. Long bones such as the femur, humerus, ulna and tibia are tubular, with a shaft and two enlarged, articular extremities. The shaft has a hollow center, the medullary cavity, and usually three external surfaces, so that on cross-section it appears triangular. Short bones such as the metatarsals are shaped like modified cubes and are mainly composed of cancellous bone with a thin outer crust of compact bone. Flat bones such as a rib or the scapula are a sandwich of cancellous bone between two layers of compact bone. The function of many flat bones is to form a protective wall around a body cavity. Irregular bones such as the vertebrae do not belong to the other categories because of their irregular shape, and they are mainly composed of cancellous bone with a covering of compact bone. Sesamoid bones such as the patella are small seed-shaped bones that serve to prevent friction where a tendon passes over a bone. Sutural bones are small, irregularly shaped bones found in the sutures of the cranium, especially in the parietal bones. As many as 172 may be found in a skull, but they are not counted among the 206 bones of the adult human skeleton. They are not named, as their number, shape and placement vary.
Individual markings can be used to identify specific bones within each category. They include elevations or projections, such as ramus, process, head, trochanter, tuberosity, trochlea, tubercle, facet, spine, crest and line, and depressions or holes, such as fossa, sulcus, foramen, alveolus, fissure, meatus, antrum and sinus.
Joints
Articulations exist where bones contact one another, and there are several types. Some, such as those between bones in the adult skull, do not allow movement. Others allow slight movement, such as the symphysis pubis, which can move slightly during childbirth. Still others, such as the hips, ankle and wrist, allow full movement, and most joints in the body are of this type.
Synarthroses allow little or no movement and include the sutures of the skull, the gomphoses at the jaw and teeth interface, and the synchondroses of the epiphyseal plate. Amphiarthroses allow slight movement and include the syndesmoses at the radius–ulna interface and the symphyses, such as the pubic bones. Diarthroses are highly mobile, and almost all joints in the body belong to one of their six types: gliding at the rib–vertebra interface, hinge at the elbow and ankle, pivot at the axis and atlas, condyloid at the metacarpals and phalanges, saddle at the base of the thumb, and ball and socket at the hip and shoulder.
Differences between male and female skeletons
Male and female skeletons differ in several ways. Male bones are generally heavier and thicker than female bones of the corresponding size. The male pelvis is deep and narrow, whereas the female pelvis is broad and shallow to facilitate child-bearing. Because of the shape of the pelvis, the hip joint in women must compensate, and female skeletons are slightly knock-kneed.
Bone growth and remodelling
Bones must grow longer and wider during the growth period of life, and both child and adult bone is constantly being reformed and remodeled. Two cell types make this possible. Osteoclasts are bone-absorbing cells found in bone tissue, and they break down bone organs. Osteoblasts are involved in the production of new bone tissue, and they form bone cells.
Cartilage and how it differs from bone
Cartilage is a firm and elastic type of dense connective tissue. Although cartilage often covers the ends of long bones and the human fetal skeleton is first composed of cartilage, cartilage is not a precursor to bone. Cartilage resembles bone, but it differs from it in having more matrix than cells. In bone, collagenous fibers are embedded in a calcified cement substance, and in cartilage they are embedded in a firm gel. Chondrocytes are mature cartilage cells, embedded in lacunae within the cartilage matrix.