The framework upon which the rest of the body is built up. The bones are generally called the skeleton, though this term also includes the cartilages which join the ribs to the breastbone, protect the larynx, etc.
Bone is composed partly of fibrous tissue, partly of bone matrix comprising phosphate and carbonate of lime, intimately mixed together. The bones of a child are about two-thirds fibrous tissue, whilst those of the aged contain one-third, explaining the toughness of the former and the brittleness of the latter.
The shafts of the limb bones consist of dense bone, a hard tube surrounded by a membrane (the periosteum) and enclosing a fatty substance (the BONE MARROW); and of cancellous bone, which forms the short bones and the ends of long bones, in which a fine lace-work fills up the whole interior, enclosing marrow in its meshes. Even the densest bone is tunnelled by fine canals (Haversian canals) in which run small blood vessels, nerves and lymphatics, for the maintenance and repair of the bone. Around these Haversian canals the bone is arranged in circular plates called lamellae, separated from one another by clefts (lacunae), in which single bone-cells are contained.
Bones grow in thickness from the fibrous tissue and lime salts laid down by cells in their substance. The long bones grow in length from a plate of cartilage (epiphyseal cartilage) which runs across the bone about 1.5 cm or more from its ends, and which on one surface is also constantly forming bone until the bone ceases to lengthen at about the age of 16 or 18. Epiphyseal injury in children may lead to diminished growth of the limb.
is by cells called osteoblasts, which manufacture strands of fibrous tissue, between which bone earth is later deposited; while other cells, known as osteoclasts, dissolve and break up dead or damaged bone. When a fracture has occurred, and the broken ends have been brought into contact, these are surrounded by a mass of blood at first; this is partly absorbed and partly organised by these cells, first into fibrous tissue and later into bone. The mass surrounding the fractured ends is called callus. This is gradually smoothed away, restoring the bone's original shape. If the ends have not been brought accurately into contact, a permanent thickening results.
Apart from the structural varieties, bones fall into four classes: (a) long bones like those of the limbs; (b) short bones composed of cancellous tissue, like those of the wrist and the ankle; (c) flat bones like those of the skull; (d) irregular bones like those of the face or the vertebrae of the spinal column (backbone).
consists of more than 200 bones. It is divided into an axial part, comprising the skull, the vertebral column, the ribs with their cartilages, and the breastbone; and an appendicular portion comprising the four limbs. The hyoid bone in the neck, together with the cartilages protecting the larynx and windpipe, are called the VISCERAL skeleton.
The skull consists of the cranium, which has eight bones; occipital, two parietal, two temporal, one frontal, ethmoid, and sphenoid; and of the face, which has 14 bones; two maxillae or upper jaw-bones, one mandible or lower jaw-bone, two malar or cheek bones, two nasal, two lacrimal, two turbinal, two palate bones, and one vomer bone. (For further details, see SKULL.) The vertebral column consists of seven vertebrae in the cervical region (neck), 12 dorsal vertebrae, five vertebrae in the lumbar or loin region, the sacrum or sacral bone (a mass formed of five vertebrae fused together), and finally the coccyx (four small vertebrae representing the tail of lower animals). Between each pair of vertebrae is a disc of cartilage. Each vertebra has a solid part, the body in front, and behind this a ring of bone, the series of rings one above another forming a bony canal through which runs the spinal cord to pass through an opening in the skull at the upper end of the canal and there join the brain. (For further details, see SPINAL COLUMN.) The ribs – 12 on each side – are attached behind to the 12 dorsal vertebrae, while in front they end a few inches away from the breastbone, but are continued forwards by cartilages. The breastbone, or sternum, is shaped something like a short sword, about 15 cm (6 inches) long, and rather over 2·5 cm (1 inch) wide.
The upper limb consists of the shoulder, upper arm, forearm, and wrist with the hand, separated from each other by joints. In the shoulder lie the clavicle (collar-bone), and the scapula (shoulder-blade). In the upper arm is a single bone, the humerus. In the forearm are two bones, the radius and ulna; the radius rotates around the ulna so that the hand can turn the hand palm upwards and back downwards (supination and pronation). In the carpus or wrist are eight small bones: the scaphoid, lunate, triquetral, pisiform, trapezium, trapezoid, capitate and hamate. In the hand proper are five bones called metacarpals, upon which are set the four fingers, each containing the three bones known as phalanges, and the thumb with two phalanges.
The lower limb consists of the hip-bone and three segments – the thigh, leg and foot. The hip-bone is a large flat bone made up of three bones – the ilium, the ischium and the pubis – fused together, and forms the side of the pelvis which encloses some of the abdominal organs. The thigh contains the femur, and the leg contains two bones – the tibia and fibula. In the ankle and first part of the foot are seven tarsal bones: the talus; the calcaneus or heel-bone; the navicular; the lateral, intermediate and medial cuneiforms; and the cuboid. Finally, as in the hand, there are five metatarsals and 14 phalanges, of which the great toe has two, the other toes three each.