1. the hard, rigid form of connective tissue constituting most of the skeleton of vertebrates, composed chiefly of calcium salts.
any distinct piece of the skeleton of the body. See anatomic Table of Bones in the Appendices for regional and alphabetical listings of bones, and see color plates 1 and 2. Called also os
. adj., adj
There are 206 separate bones in the human body. Collectively they form the skeletal system
, a structure bound together by ligaments at the joints and set in motion by the muscles, which are secured to the bones by means of tendons. Bones, ligaments, muscles, and tendons are the tissues of the body responsible for supporting and moving the body.
Some bones have a chiefly protective function. An example is the skull, which encloses the brain, the back of the eyeball, and the inner ear. Some, such as the pelvis, are mainly supporting structures. Other bones, such as the jaw and the bones of the fingers, are concerned chiefly with movement. The bone marrow
in the center manufactures blood cells
. The bones themselves act as a storehouse of calcium
, which must be maintained at a certain level in the blood for the body's normal chemical functioning.
Structure and Composition. Bone is not uniform in structure but is composed of several layers of different materials. The outermost layer, the periosteum, is a thin, tough membrane of fibrous tissue. It gives support to the tendons that secure the muscle to the bone and also serves as a protective sheath. This membrane encloses all bones completely except at the joints where there is a layer of cartilage. Beneath the periosteum lie the dense, hard layers of bone tissue called compact bone. Its composition is fibrous rather than solid and it gives bone its resiliency. Encased within these layers is the tissue that makes up most of the volume of bone, called cancellous or spongy bone because it contains little hollows like those of a sponge. The innermost portion of the bone is a hollow cavity containing marrow. Blood vessels course through every layer of bone, carrying nutritive elements, oxygen, and other products. Bone tissue also contains a large number of nerves. The basic chemical in bone, which gives bone its hardness and strength, is calcium phosphate.
Development. Cartilage forms the major part of bone in the very young; this accounts for the great flexibility and resiliency of the infant skeleton. Gradually, calcium phosphate collects in the cartilage, and it becomes harder and more brittle. Some of the cartilage cells break loose, so that channels develop in the bone shaft. Blood vessels enter the channels, bearing with them small cells of connective tissue, some of which become osteoblasts, cells that form true bone. The osteoblasts enter the hardened cartilage, forming layers of hard, firm bone. Other cells, called osteoclasts, work to tear down old or excess bone structure, allowing the osteoblasts to rebuild with new bone. This renewal continues throughout life, although it slows down with age.
Cartilage formation and the subsequent replacement of cartilage by hard material is the mechanism by which bones grow in size. During the period of bone growth, cartilage grows over the hardened portion of bone. In time, this layer of cartilage hardens as calcium phosphate is added, and a fresh layer grows over it, and it too hardens. The process continues until the body reaches full growth. Long bones grow in length because of special cross-sectional layers of cartilage located near the flared ends of the bone. These harden and new cartilage is produced by the same process as previously described.
, a break in the bone, is the most common injury to the bone; it may be closed, with no break in the skin, or open, with penetration of the skin and exposure of portions of the broken bone. osteoporosis
is excessive brittleness and porosity of bone in the aged. osteomyelitis
is a bone infection similar to a boil on the skin, but much more serious because blood supply to bone is less exquisite than that to other body organs and bone metabolizes more slowly, so that the infection can destroy the bone and invade other body tissues. osteomalacia
is the term used for rickets
when it occurs in adults. In these diseases there is softening of the bones, due to inadequate concentration of calcium or phosphorus in the body. The usual cause is deficiency of vitamin D, which is required for utilization of calcium and phosphorus by the body. In osteitis fibrosa cystica
, bone is replaced by fibrous tissue because of abnormal calcium metabolism. The condition usually is due to overactivity of the parathyroid glands. osteoma
refers to abnormal new growth, either benign or malignant, of the tissue of the bones. Although it is not common, it may occur in any of the bones of the body, and at any age.
alveolar bone the thin layer of bone making up the bony processes of the maxilla and mandible, surrounding and containing the teeth; it is pierced by many small blood vessels, lymphatic vessels, and nerves.
cancellated bone (cancellous bone) bone composed of thin intersecting lamellae, usually found internal to compact bone.
cartilage bone bone developing within cartilage, ossification taking place within a cartilage model, as opposed to membranous bone.
cheek bone zygomatic bone.
compact bone bone substance that is dense and hard.
cortical bone the compact bone of the shaft of a bone that surrounds the marrow cavity.
the bones that constitute the cranium
, including the occipital, sphenoid, temporal, parietal, frontal, ethmoid, lacrimal, and nasal bones, the inferior nasal concha, and the vomer. Some authorities also include the maxilla, zygomatic bone, and palatine bone. See anatomic Table of Bones in the Appendices.
ethmoid bone the sievelike bone that forms a roof for the nasal fossae and part of the floor of the anterior cranial fossa. See anatomic Table of Bones in the Appendices.
facial b's the bones that form the skeleton of the face, including the hyoid, palatine, and zygomatic bones, the mandible, and the maxilla. Some authorities include the lacrimal bones, nasal bones, inferior nasal concha, and vomer and exclude the hyoid bone. See anatomic Table of Bones in the Appendices.
flat bone one whose thickness is slight, sometimes consisting of only a thin layer of compact bone, or of two layers with intervening cancellous bone and marrow; usually curved rather than flat.
frontal bone the bone at the anterior part of the skull. See anatomic Table of Bones in the Appendices.
hip bone the ilium, ischium, and pubis as a unit. See anatomic Table of Bones in the Appendices.
a horseshoe-shaped bone at the base of the tongue. See anatomic Table of Bones in the Appendices. Called also lingual bone
incisive bone the portion of the maxilla bearing the incisors; developmentally, it is the premaxilla, which in humans later fuses with the maxilla, but in most other vertebrates persists as a separate bone.
either the mandible
(lower jaw) or the maxilla
(upper jaw). See anatomic Table of Bones in the Appendices.
long bone one whose length far exceeds its breadth and thickness.
membrane bone (membranous bone) bone that develops within a connective tissue membrane, in contrast to cartilage bone.
occipital bone the bone constituting the back and part of the base of the skull. See anatomic Table of Bones in the Appendices.
parietal bone one of two bones forming the sides and roof of the cranium. See anatomic Table of Bones in the Appendices.
pneumatic bone bone that contains air-filled spaces.
rider's bone localized ossification sometimes seen on the inner aspect of the lower end of the tendon of the adductor muscle of the thigh in horseback riders.
short bone one of approximately equal length, width, and thickness.
any of the variable and irregularly shaped bones in the sutures between the bones of the skull. Called also wormian bone
one of two bones forming part of the lateral and inferior surfaces of the skull and containing the organs of hearing
. See anatomic Table of Bones in the Appendices.
a nasal concha
zygomatic bone the quadrilateral bone that forms a cheek. See anatomic Table of Bones in the Appendices.
Osseous tissue, a specialized form of dense connective tissue consisting of bone cells (osteocytes) embedded in a nonliving matrix. Bone matrix is made of calcium carbonate, calcium phosphate, and collagen fibers. Synonym: os.
PARTS OF A LONG BONE
A unit of the skeleton; the human skeleton has 206 bones. Bones surround and protect some vital organs, and give points of attachment for the muscles, serving as levers and making movement possible. In the embryo, the bones of the skull are first made of fibrous connective tissue, which is gradually replaced by bone matrix. The remainder of the skeleton is first made of hyaline cartilage, which is also replaced by bone matrix, beginning during the third month of gestation. The outer surface of a bone is compact bone, and the inner more porous portion is cancellous (spongy) bone. The shafts of long bones are made of compact bone that surrounds a marrow canal. Compact bone is made of haversian systems, which are precise arrangements of osteocytes, blood vessels, and lymphatics within the bony matrix. All of these contribute to the maintenance and repair of bone. The periosteum is the fibrous connective tissue membrane that covers a bone. It has blood vessels that enter the bone, and it provides a site of attachment for tendons and ligaments. Bones are classified according to shape as long, short, flat, or irregular. In the elderly, esp. women, osteoporosis may develop, a condition in which bones become brittle and break easily. See: illustration
for names of principal bones
The bony tissue or process of the maxilla or mandible that supports the teeth. Synonym: alveolar process
The basilar process of the occipital bone.
Bone that is abnormally fragile, as in osteogenesis imperfecta.
A spongy bone in which the matrix forms connecting bars and plates, partially enclosing many intercommunicating spaces filled with bone marrow. Synonym: spongy bone
One of the eight wrist bones, which are aligned in two rows. The proximal row contains (from the thumb to the little finger) the scaphoid, lunate, triquetral, and pisiform bones. The distal row contains (from thumb to little finger) the trapezium, trapezoid, capitate, and hamate bones.
A bone formed by endochondral ossification developing from the primary centers of bone formation. Synonym: endochondral bone
cavalry boneRider's bone.
The hard, dense bone made of haversian systems that forms the surface layer of all bones and the shafts of long bones, in contrast to spongy bone that forms the bulk of the short, flat, and irregular bones and the ends of long bones.
A bone that forms a part of the medial portion of the acetabulum during fetal development. It subsequently fuses with the pubis.
A bone of the skull or brain case.
The outer bone of the instep bones of the foot that articulates posteriorly with the calcaneus and anteriorly with the fourth and fifth metatarsals.
One of the bones of the internal, middle, and external tarsus.
dermal boneMembrane bone.
One of the ossicles of the tympanic cavity: the malleus, incus, and stapes. See: ear for illus.
endochondral boneCartilage bone.
A complex thin-walled bone, roughly cuboidal in shape, located in the middle of the skull above the nasal cavities and below the anterior fossa of the cranial cavity. Its flat upper surface is the cribriform plate, which forms much of the roof of the nasal cavities; its upper surface has a midline bony keel that projects up into the cranial cavity and on both sides of which are perforated valleys through which the olfactory nerves project up from the olfactory epithelium. In the midline under the cribriform plate is a mirror-image (to the crista galli) keel, the perpendicular plate, which projects down between the nasal cavities as part of the bony nasal septum. The right and left sides of the ethmoid bone are the ethmoidal labyrinths, composed of ethmoidal air cells; the inner surfaces of the labyrinths form the middle nasal conchae, while the lateral surfaces form the orbital plates, which are part of the mosaic of bones that form the inner walls of the orbits.
The forehead bone.
A colloquial term for the groove along the inner back side of the elbow (behind and underneath the medial epicondyle of the humerus) in which the ulnar nerve runs. Pressure on the groove compresses the ulnar nerve, producing a tingling discomfort on the inside of the forearm as well as the 4th and 5th fingers.
greater multangular bone
The first or outermost of the distal row of carpal bones. Synonym: trapezium
The most medial wrist (carpal) bone in the distal row. It has a hooked process on its palmar side. The hamate articulates with the 4th and 5th metacarpals. Synonym: hamatum
; os hamatum
; unciform bone
hip boneInnominate bone.
The horseshoe-shaped bone at the base of the tongue. It is mobile and its ends hang by the stylohyoid ligaments from the styloid process on each side of the base of the skull. The hyoid bone is suspended by many muscles (the hyoid muscles): the suprahyoid muscles (geniohyoid, mylohyoid, digastric, and hyoglossus) attach the hyoid bone to the mandible and the floor of the mouth; the infrahyoid muscles (omohyoid, sternohyoid, and thyrohyoid) attach it to the larynx and the thoracic cage. The hyoid bone anchors and moves with the jaw, tongue, pharynx, and larynx. See: illustration
The part of the maxilla bearing the incisor teeth.
The hip bone or os coxae, composed of the ilium, ischium, and pubis. It is united with the sacrum and coccyx by ligaments to form the pelvis. Synonym: pelvic bone
; os coxae.
The squamous portion of the occipital bone.
The alveolar bone between the roots of multirooted teeth.
intramembraneous boneMembrane bone.
ivory boneMarble bone.
A thin, irregularly shaped bone on the medial side of the orbit.
lesser multangular bone
lunate boneSemilunar bone.
A four-pointed bone on each side of the face, uniting the frontal and superior maxillary bones with the zygomatic process of the temporal bone. Synonym: cheekbone
; zygomatic bone
An abnormally calcified bone with a spotted appearance on a radiograph. Synonym: ivory bone See: osteopetrosis
Any of the bones of the metatarsus. See: foot for illus.
Bone formed within embryonic fibrous connective tissue, in which fibroblasts differentiate into osteoblasts. Such bone is formed without a cartilage model and includes the bones of the face and cranium. Synonym: dermal bone; intramembraneous bone
Bone appearing as small pieces fitted together, characteristic of Paget's disease.
Either of the two small bones forming the bridge of the nose.
The bone that forms the lower, posterior skull; it articulates with the parietal and temporal bones anteriorly, and the atlas inferiorly.
The rounded end of the long process of the incus, a middle ear ossicle. It probably represents a secondary ossification center in the long or lenticular process.
palate bonePalatine bone.
One of the bones forming the posterior part of the hard palate and lateral nasal wall between the interior pterygoid plate of the sphenoid bone and maxilla. Synonym: palate bone
One of two bones that together form the posterior roof and sides of the skull.
pelvic boneInnominate bone.
Bone formed beneath the perichondrium.
Bone formed by osteoblasts of the periosteum.
ping pong bone
A thin shell of osseous tissue covering a giant-cell sarcoma in a bone.
The lower anterior part of the innominate bone. It is a separate bone at birth; it begins to fuse with the ischium by age 8, and it is fully fused with the ischium and ilium by 16-18 years of age. Synonym: os pubis
Any bone that develops within cartilage.
Ossification of the distal end of the adductor muscles of the thigh, as may be seen in horseback riders. Synonym: cavalry bone
The largest carpal bone in the first row of wristbones. It is on the thumb side of the hand and articulates directly with the radius. Synonym: os scaphoideum
Crescent-shaped bone of the carpus. Synonym: lunate bone
A type of short bone occurring in the hands and feet and embedded in tendons or joint capsules.
Soemmering's bone See: Soemmering's bone
The large bone at the base of the skull that has the ethmoid bone in front of it, the occipital bone behind it, and the parietal and temporal bones at the sides. It is shaped like a large moth. Its two broad, curved wings form the front walls of the middle cranial fossae, and its two “tails, ” the pterygoid processes, which hang in front of the neurocranium in the pterygoid fossa behind the facial skeleton. Between the wings, in the center of the body of the sphenoid bone, there is a deep, concave pocket (the sella turcica), in which the pituitary gland lies.
spongy boneCancellous bone.
The upper anterior portion of temporal bone.
sutural boneWormian bone.
A flat bone, or one with two compact bone portions enclosing a center of spongy bone.
One of the seven bones of the ankle, hind-foot, and midfoot, consisting of the talus, calcaneus, navicular, cuboid, and three cuneiform bones.
A bone on both sides of the skull at its base. It is composed of squamous, mastoid, and petrous portions, the latter enclosing the receptors for hearing and equilibrium. Synonym: os temporale See: Arnold's canal; mastoid; petrosa; styloid process
The second bone in the distal row of carpal bones. It lies between the trapezium and capitate bones.
The third carpal bone in the proximal row, enumerated from the radial side. Synonym: triquetrum
The hamate bone, the hook-shaped bone on the ulnar side of the distal row of the carpus. Synonym: os hamatum
One of the small, irregular bones found along the cranial sutures. Synonym: sutural bone
Embryonic or rapidly growing bone characterized microscopically by a prominent fibrous matrix.
The cheekbone; the bone on either side of the face below the eye. Synonym: malar bone