Bony substance

Bony substance comprises two differentiated structures: the outer compact cortical layer and the inner spongy material (lamellar or cancellous bone).

The compact cortical layer (substantia compac-ta, corticalis) forms a layer of thickened bone material at the surface of the bone, which is particularly thick in the middle part of a tubular bone and thinner at the joints. At the surface of the cortical layer, there are small openings to the Volkmann and Haversian canals, in which blood vessels and nerves run from the periosteum through the bone into the bone marrow.

The spongy material (substantia spongiosa) forms a framework of fine trabeculae, whose holes are filled with soft bone marrow. There are no trabeculae in the middle section of a long or tubular bone, so that a uniform bone cavity is produced (cavum medullare).The fine trabeculae (spongy trabeculae) are arranged according to lines of pressure and traction (Fig 2-13) with two guiding principles: (1) the minimum and maximum rule, whereby the greatest work is achieved with the least effort; and (2) the form and function rule, according to which the function determines the form and form influences function. Bone mass is only built up at points where mechanical stresses are exerted. Where there is no mechanical strain, this substance is absent. Owing to the special arrangement of the spongy trabeculae along the lines of mechanical force, the bone becomes pliable in a specific way and equally resistant to bending. This is because a bone only bends in response to stress as much as the spongy trabeculae will allow, and this provides the stimulus to strengthening the spongy trabeculae in their particular form.

Bone marrow

Bone marrow (medulla osseum) is a soft, jellylike substance that is found both in the gaps between the spongy trabeculae and in the bone cavity of long bones. A distinction is made between red and yellow bone marrow. The red, blood-forming marrow in adults is only found in the gaps between the spongy trabeculae of flat bones (eg, ribs, cranial bones, sternum, vertebral bodies, and wrist bones). Yellow bone marrow is mainly found in the medullary cavity of the long bones. The bone marrow and bony substance are nourished via blood vessels. These vessels run from the periosteum through to the bone cavity via a system of Volkmann canals, which in turn branch into Haversian canals.

Form and function of bone

Bone has the following functions:

• To support the soft tissues
• To form attachments for the active locomotor system (the muscles)
• To protect sensitive organs such as the heart, lungs, and brain
• To p roduce red bl ood cel l s (as the bl ood-forming center)

The passive locomotor system comprises bones and their joint cartilages. The bones are connected to each other via joints.

The form of bone is determined by its specific function (Figs 2-14 and 2-15). This means that pressure loading from vessels, nerves, or muscles can cause the formation of, for example, depressions, grooves, and cavities, while tensile strain may result in the formation of processes, spurs, or tubercles on the relevant bones.

A distinction is made between the following forms of bone:

• Long tubular bones (ossa longa) (Fig 2-16) or supporting bones are the long bones of the extremities, which are made up of a tubular middle section (diaphysis) and the two thickened ends covered with joint cartilage (epiphyses). There is a region of lengthwise growth (metaphysis) between the diaphysis and the epiphyses. The diaphysis ossifies, unlike the two ends, which remain cartilaginous at first. Tubular long bones have a medullary cavity in their middle section, which is filled with red, blood-forming bone marrow in young people and with yellow (fat) marrow in older people.
• Flat bones (ossa plana) comprise two layers of compact bone with spongy material lying between the layers. The skull bones, hip bones, and shoulder blades are typical flat bones, which surround the cavities to protect sensitive organs and are therefore also known as protective bones.
• Short bones (ossa brevia) are the carpal and tarsal bones of the hands and feet, which mainly consist of spongy bone and are surrounded by a thin cortical layer.
• Irregular bones containing air perform protective, supportive, and locomotor functions, as shown by the example of the craniofacial bones.
• The cavities of spongy bones, epiphyses, and long bones as well as short and flat bones contain red, blood-forming bone marrow in which the red blood cells, granulocytes, and platelets are formed.

If the bone marrow has to be clinically examined, a biopsy is taken from the sternum, and the bone marrow specimen obtained is stained.