The skeleton of a human adult consists of 206 individual bones, but this chapter covers only the major bones. For anatomical purposes, the human skeleton is divided into the axial skeleton (distinguished with bone color in Figure Anterior view of the skeleton.) and the appendicular skeleton (distinguished with blue color in Figure Anterior view of the skeleton.).
The axial skeleton protects internal organs and provides central support of the body around which other parts move. It consists of the bones of the head, chest, and spine. The appendicular skeleton enables the body to move. It consists of the bones of the shoulders, arms, hips, and legs. The ability to walk, run, or catch a ball is possible due to the movable joints of the limbs.
Anterior view of the skeleton.
Structure and Function of Bones
To understand the skeletal system, it is important to know the types and names of major bones, their functions, and where they are located.
There are four principal types of bones: long, short, flat, and irregular.
The long bones of the extremities are the strongest bones of the arms and legs. The cube-shaped short bones include the bones of the ankles, wrists, and toes. Flat bones are the broad bones found in the skull, shoulder, and ribs. Irregular bones have varied shapes and sizes and are commonly clustered, such as the bones of the vertebrae and certain bones of the ears and face.
Typically, long bones are found in the extremities of the body.
The main elongated portion of such a bone, the (1) diaphysis, is composed of several tissue layers: the thin fibrous outer membrane, the(2) periosteum; the thick layer of hard (3) compact bone; and the inner 4) medullary cavity. Label the parts of the long bone in Figure Longitudinal section of a long bone (femur) and interior bone structure.
The two ends of bones, the (5) distal epiphysis and (6) proximal epiphysis, have a bulbous shape to provide space for muscle and ligament attachments near the joints.
There are two kinds of bone tissue based on porosity, and most bones have both types. Compact (dense) bone tissue is the hard, outer layer; (7) spongy (cancellous) bone tissue is the porous, highly vascular inner portion. Compact bone tissue is covered by periosteum that serves as a place of attachment for muscles, provides protection, and gives durable strength to the bone.
The spongy bone tissue makes the bone lighter and provides a space for bone marrow where blood cells are produced. Label the spongy bone in Figure Longitudinal section of a long bone (femur) and interior bone structure., and note the position and structure of compact and spongy bone.
In Figure Longitudinal section of a long bone (femur) and interior bone structure., observe how the diaphysis forms a cylinder that surrounds the medullary cavity. In adults, the medullary cavity contains fat yellow marrow, so named because of the large amounts of fat it contains.
Longitudinal section of a long bone (femur) and interior bone structure.
The peri/oste/um, as illustrated in Figure Longitudinal section of a long bone (femur) and interior bone structure., covers the entire surface of the bone. Its blood vessels supply nutrients, and its nerves signal pain. In growing bones, the inner layer contains bone-forming cells known as oste/o/blasts.
Because blood vessels and oste/o/blasts are located here, the peri/oste/um provides a means for bone repair and general bone nutrition. Bones that lose peri/oste/um through injury or disease usually scale or die. As discussed earlier, the peri/oste/um also provides a point of attachment for muscles.
In an adult, production of red blood cells (erythr/o/poiesis) occurs in red bone marrow. Red bone marrow is also responsible for formation of white blood cells (leuk/o/poiesis) and platelets. Cartilage, which is more elastic than bone, composes parts of the skeleton. It is found chiefly in the joints, thorax, trachea, and nose.
Oste/algia means pain in a bone.
Bone is living tissue composed of oste/o/cytes, blood vessels, and nerves.
Dist/al is a directional word that means farthest from the point of attachment to the trunk, or far from the beginning of a structure. Proxim/al is a directional word that means near the point of attachment to the trunk, or near the beginning of a structure.
Milk is a good source of vitamin D. Deficiency of this vitamin results in a softening and weakening of the skeleton, causing pain and bowing of the bones.
Oste/o/malacia is the result of inadequate amounts of phosphorus and calcium in blood for mineralization of the bones. It may be caused by a diet lacking these minerals, deficiency in vitamin D, or a metabolic disorder that causes malabsorption of minerals.
A form of oste/o/malacia known as rickets is seen in infants and children in many underdeveloped countries. It is a result of vitamin D deficiency. Symptoms of rickets include soft, pliable bones that cause such deformities as bowlegs and knock-knees.
Calcium provides bone strength that is needed for its supportive functions. Many children in underdeveloped countries have rickets because of inadequate milk supply.
Radi/o/logy, initially widely called roentgen/o/logy, was developed after discovery of an unknown ray in 1895 by Wilhelm Roentgen, who called his discovery a roentgen (x-ray). Occasionally you still may see words with roentgen/o, but radi/o is the preferred term used in the context of medical imaging today.
Radi/o/logy is the branch of medicine concerned with radioactive substances. It is used to diagnose path/o/log/ical conditions of the skeletal system. A physician who specializes in the study of x-rays is called aradi/o/logist
Radiation is used for diagnostic and therapeutic purposes. Radiation therapy, also called radi/o/therapy, is treatment of diseases using either an external source of high-energy rays or internally implanted radioactive substances. These rays and substances are effective in damaging cancer cells and halting their growth.
A myel/o/gram, a radiograph of the spinal canal after injection of a contrast medium, is used to identify and study spinal lesions caused by trauma or disease.
To allow for body movements, bones must have points where they meet (articulate). These articulating points form joints that have various degrees of mobility. Some are freely movable (diarthroses), others are only slightly movable (amphiarthroses), and the remaining are totally immovable (synarthroses). All three types are necessary for smooth, coordinated body movements.
Arthr/o/scopy is the visual examination of the interior of a joint performed by inserting an endo/scope through a small incision. Arthr/o/ scopy is performed to repair and remove joint tissue, especially of the knee, ankle, and shoulder. (See Figure Arthroscopy.)
Total hip arthr/o/plasty is a surgical procedure to replace the femur and acetabulum with metal components. The acetabulum is plastic coated to avoid metal-to-metal articulating surfaces. (See Figure Total hip arthroplasty.)
Just as a piece of machinery is lubricated by oil, joints are lubricated by synovial fluid. The fluid is secreted within the synovial membranes. A person with arthr/itis suffers, not only from an inflammation of the joints, but also from arthr/algia.
Total hip arthroplasty. (A) Arthritis of the right hip. (B) Total hip arthroplasty of arthritic hip.
Combining Forms Related to Specific Bones
Cephal/o/dyniais the medical term for a headache.
Anterior view of the skeleton.
Encephal/itis is usually caused by viruses (for example, arborvirus,herpesvirus). Less commonly, it may occur as a component of rabies and acquiredimmune deficiency syndrome (AIDS). It may also occur as a result of systemic viral diseases, such as influenza, rubella, and chickenpox.
Fractures and Repairs
A fracture is a break or crack in the bone. Fractures are defined according to the type and extent of the break. A (1) closed fracture means the bone is broken with no open wound, and surrounding tissue damage is minimal. An (2) open fracture, also called a compound fracture, means the broken end of a bone pierces the skin, creating an open wound. In such a fracture, there may be extensive damage to surrounding blood vessels, nerves, and muscles. Label the closed and open fractures in Figure Types of fractures.
Types of fractures.
In addition to determining the extent of a break in a fracture, there are many different types of bone fractures, some of which are discussed here. A (3) greenstick fracture means there is an incomplete break of a soft bone, which means the bone is partially bent and partially broken.
These fractures usually occur in children because their growing bones are soft and tend to splinter, rather than break completely. A (4) comminuted fracture occurs when the bone is broken into pieces. In an (5) impacted fracture, the broken ends of a bone are forced into one another; many bone fragments may be created by such a fracture.
A (6) complicated fracture involves extensive soft tissue injury, such as when a broken rib pierces a lung. A (7) Colles fracture is a break of the lower end of the radius, which occurs just above the wrist. It causes displacement of the hand and usually occurs as a result of flexing a hand to cushion a fall. An (8) incomplete fracture is when the line of fracture does not include the whole bone. Label and study the different types of fractures in Figure Types of fractures
The vertebr/al or spin/al column supports the body and provides a protective bony canal for the spinal cord. (See Figure Vertebral column, lateral view, with regions of the spine shown with normal curves.)
Vertebral column, lateral view, with regions of the spine shown with normal curves.
The vertebr/al column, also called the spin/al column or backbone, is composed of 26 bones known as vertebrae (singular, vertebra). There are five regions of these bones in the vertebr/al column, each of which derives its name from its location along the length of the spin/al column. Seven(2) cervical vertebrae form the skeletal framework of the neck.
The firstcervic/al vertebra is called the (3) atlas and supports the skull. The second,the (4) axis, enables the skull to rotate on the neck. Label these structures in Figure Vertebral column, lateral view, with regions of the spine shown with normal curves.
Twelve (5) thoracic vertebrae support the chest and serve as a point of articulation for the ribs. The next five vertebrae are the (6) lumbar vertebrae. These are situated in the lower back and carry most of the weight of the torso. Label these structures in Figure Vertebral column, lateral view, with regions of the spine shown with normal curves.
Examine the position of the five lumbar vertebrae in Figure Vertebral column, lateral view, with regions of the spine shown with normal curves. These are designated as L1 to L5 in medical reports. An obese person with weak abdominal muscles tends to experience pain in the lower back area, or L1 to L5.
Below the lumbar vertebrae are five sacral vertebrae that are fused into a single bone in the adult. The single bone is known as the (7) sacrum and the tail of the vertebral column, the (8) coccyx. To designate the exact position of abnormalities on the sacrum, the label S1 to S5 is used. The first vertebra of the sacrum is designated as S1.
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