Bone structure (see Fig. 1-L)

Bone structure (see Fig. 1-L)

Bone consists of an external layer of dense compact bone which varies in thickness

according to the forces applied to it. This encloses the cancellous (spongy) bone of

more loosely arranged bony plates and spicules which are aligned according to the

mechanical stresses experienced. The cavities between plates are occupied by marrow

(marrow spaces). Cancellous bone forms the bulk of short bones and the extremities of

long bones. The hollow shaft of long bones is termed the medullary cavity and contains

marrow. Much of the bone marrow in the body is red marrow in young animals

(haemopoietic; blood forming), but this is largely replaced by adipose tissue to form

yellow marrow in older animals.

Some regions, such as the sternum, vertebrae, ribs, skull, pelvis and proximal epiphyses

of the femur and humerus, contain red marrow throughout life. The wing of the ileum

and the sternum provide a convenient location for its collection.

The periosteum envelops the outer surface of the bone and consists of two parts - an

outer protective layer and an inner osteogenic layer. It is critically important in repair of

bone following injury, and excessive stripping of the periosteum during surgery or

fracture is detrimental to new bone formation. The thin fibrous endosteum lines the

medullary cavity, and is important in early callus formation during fracture repair.

Figure 1-L: Sagittal section of a long bone