Bones have a number of functions, several of which are not evident in looking at the skeleton:
* To serve as a firm framework for the entire body;
* To protect such delicate structures as the brain and the spinal cord;
* To serve as levers, working with attached muscles to produce movement;
* To serve as a storehouse for calcium salts, which may be resorbed into the blood if there is not enough calcium in the diet;
* To produce blood cells (in the red marrow).

Bone Structure

The complete bony framework of the body, known as the skeleton (Fig. 3-1), consists of 206 bones. It is divided into a central portion, the axial skeleton, and the extremities, which make up the appendicular skeleton. The bones of the skeleton can be of several different shapes. They may be flat (ribs, cranium), short (carpals of wrist, tarsals of ankle), or irregular (vertebrae, facial bones). The most familiar shape, however, is the long bone, the type of bone that makes up almost all of the skeleton of the arms and legs. The long narrow shaft of this type of bone is called the diaphysis. At the center of the diaphysis is a medullary cavity, which contains bone marrow. The long bone also has two irregular ends, a proximal and a distal epiphysis (Fig. 3-2).
The Skeleton
The structure of a long bone
Figure 3-2 The structure of a long bone.
Figure 3-1 The skeleton. The axial skeleton is shown in yellow; the appendicular, in blue.
Bone Tissue Bones are not lifeless. Even though the spaces between the cells of bone tissue are permeated with stony deposits of calcium salts, the bone cells themselves are very much alive. Bones are organs, with their own system of blood vessels, lymphatic vessels, and nerves. There are two types of bone tissue, also known as osseous tissue. One type is compact bone, which is hard and dense (Fig. 3-3). This tissue makes up the main shaft of a long bone and the outer layer of other bones. The cells in this type of bone are located in rings of bone tissue around a central haversian canal containing nerves and blood vessels. The bone cells live in spaces (lacunae) between the rings and extend out into many small radiating channels so that they can be in contact with nearby cells. Each ringlike unit with its central canal makes up a haversian system, also known as an osteon (see Fig. 3-3 B). Forming a channel across the bone, from one side of the shaft to the other, are many perforating (Volkmann) canals, which also house blood vessels and nerves. The second type of bone tissue, called spongy, or cancellous, bone, has more spaces than compact bone. It is made of a meshwork of small, bony plates filled with red marrow. Spongy bone is found at the epiphyses (ends) of the long bones and at the center of other bones. Figure 3-4 shows a photograph of both compact and spongy tissue in a bone section.
bone tissue
Figure 3-4 Bone tissue, longitudinal section. Spongy (cancellous) bone makes up most of the epiphysis (end) of this long bone, shown by the arrows.
Bone tissue
Figure 3-3 Compact bone tissue. (A) This section shows osteocytes (bone cells) within osteons (haversian systems). It also shows the canals that penetrate the tissue. (B) Microscopic view of compact bone in cross section (300) showing a complete osteon. In living tissue, osteocytes (bone cells) reside in spaces (lacunae) and extend out into channels that radiate from these spaces.
Bone Marrow Bones contain two kinds of marrow. Red marrow is found at the ends of the long bones and at the center of other bones (see Fig. 3-2). Red bone marrow manufactures blood cells. Yellow marrow is found chiefly in the central cavities of the long bones. Yellow marrow is composed largely of fat.
Bone Membranes Bones are covered on the outside (except at the joint region) by a membrane called the periosteum (see Fig. 3-2). The inner layer of this membrane contains cells (osteoblasts) that are essential in bone formation, not only during growth but also in the repair of injuries. Blood vessels and lymphatic vessels in the periosteum play an important role in the nourishment of bone tissue. Nerve fibers in the periosteum make their presence known when one suffers a fracture, or when one receives a blow, such as on the shinbone. A thinner membrane, the endosteum, lines the marrow cavity of a bone; it too contains cells that aid in the growth and repair of bone tissue.

Bone Growth and Repair

During early development, the embryonic skeleton is at first composed almost entirely of cartilage. (Portions of the skull develop from fibrous connective tissue.) The conversion of cartilage to bone, a process known as ossification, begins during the second and third months of embryonic life. At this time, bone-building cells, called osteoblasts, become active. First, they begin to manufacture the matrix, which is the material located between the cells. This intercellular substance contains large quantities of collagen, a fibrous protein that gives strength and resilience to the tissue. Then, with the help of enzymes, calcium compounds are deposited within the matrix. Once this intercellular material has hardened, the cells remain enclosed within the lacunae (small spaces) in the matrix. These cells, now known as osteocytes, are still living and continue to maintain the existing bone matrix, but they do not produce new bone tissue. When bone has to be remodeled or repaired later in life, new osteoblasts develop from stem cells in the endosteum and periosteum. One other type of cell found in bone develops from a type of white blood cell (monocyte). These large, multinucleated osteoclasts are responsible for the process of resorption, which is the breakdown of bone tissue. Resorption is necessary for remodeling and repair of bone, as occurs during growth and after injury. Bone tissue is also resorbed when its stored minerals are needed by the body. The formation and resorption of bone tissue are regulated by several hormones. Vitamin D promotes the absorption of calcium from the intestine. Other hormones involved in these processes are produced by glands in the neck. Calcitonin from the thyroid gland promotes the uptake of calcium by bone tissue. Parathyroid hormone (PTH) from the parathyroid glands at the posterior of the thyroid causes bone resorption and release of calcium into the blood.
Formation of a Long Bone In a long bone, the transformation of cartilage into bone begins at the center of the shaft during fetal development. Around the time of birth, secondary bone-forming centers, or epiphyseal plates, develop across the ends of the bones. The long bones continue to grow in length at these centers by calcification of new cartilage through childhood and into the late teens. Finally, by the late teens or early 20s, the bones stop growing in length. Each epiphyseal plate hardens and can be seen in x-ray films as a thin line, the epiphyseal line, across the end of the bone. Physicians can judge the future growth of a bone by the appearance of these lines on x-ray films. As a bone grows in length, the shaft is remodeled so that it grows wider as the central marrow cavity increases in size. Thus, alterations in the shape of the bone are a result of the addition of bone tissue to some surfaces and its resorption from others. The processes of bone resorption and bone formation continue throughout life, more actively in some places than in others, as bones are subjected to “wear and tear” or injuries. The bones of small children are relatively pliable because they contain a larger proportion of cartilage and are undergoing active bone formation. In elderly people, there is a slowing of the processes that continually renew bone tissue. As a result, the bones are weaker and more fragile. Elderly people also have a decreased ability to form the protein framework on which calcium salts are deposited. Fractures in elderly people heal more slowly because of these decreases in bone metabolism.

Bone Markings
In addition to their general shape, bones have other distinguishing features, or bone markings. These markings include raised areas and depressions that help to form joints or serve as points for muscle attachments and various holes that allow the passage of nerves and blood vessels. Some of these identifying features are described next.
* Head a rounded, knoblike end separated from the rest of the bone by a slender region, the neck.
* Process a large projection of a bone, such as the upper part of the ulna in the forearm that creates the elbow.
* Condyle a rounded projection; a small projection above a condyle is an epicondyle.
* Crest a distinct border or ridge, often rough, such as over the top of the hip bone.
* Spine a sharp projection from the surface of a bone, such as the spine of the scapula (shoulder blade).
Depressions or Holes
* Foramen a hole that allows a vessel or a nerve to pass through or between bones. The plural is foramina.
* Sinus an air space found in some skull bones.
* Fossaa depression on a bone surface. The plural is fossae.
* Meatus a short channel or passageway, such as the channel in the temporal bone of the skull that leads to the inner ear.
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