Английский язык для медиков: конспект лекций

When classified on the basis of how they develop, bones are divided into two groups: endochondral bones and intramembraneous bones. Endochon-dral bones, such as the long bones and the bones at the base of the skull, develop from cartilage tissue. Intramembraneous bones, such as the flat bones of the roof of the skull, are not formed from cartilage but develop under or within a connective tissue membrane. Although en-dochondral bones and intramembraneous bones form in different ways, they have the same structure.

The formation of bone tissue (ossification) begins early in embryo-logical development, when the embryo is two months old. The bones continue to grow during childhood and adolescence, reaching their full size when the person is about 25.

Most adult bone is composed of two types of tissue: an outer layer of compact bone and an inner layer of spongy bone. Compact bone is strong and dense. Spongy bone is light and porous and contains bone marrow. The amount of each type of tissue varies in different bones.

The flat bones of the skull consist almost entirely of compact bone, with very little spongy tissue. In a long bone, such as the thigh bone, the shaft, called the diaphysis, is made up largely of compact bone. While the ends, called epyphyses, consist mostly of spongy bone. In a long bone, marrow is also present inside the shaft, in a cavity called the medullary cavity.

Surrounding every bone, except at the surface where it meets another bone, is a fibrous membrane called the periosteum.

The outer layer of the periosteum consists of a network of densely packed collagen fibres and blood vessels. This layer serves for the attachment of tendons, ligaments, and muscles to the bone and is also important in bone repair.

The inner layer of the periosteum has many fibres, called fibres of Sharpey, which penetrate the bone tissue, anchoring the periosteum to the bone. The inner layer also has many bone-forming cells, or osteo-blasts, which are responsible for the bone's growth in diameter and the production of new bone tissue in cases of fracture, infection.

In addition to the periosteum, all bones have another membrane, the endosteum. It lines the marrow cavity as well as the smaller cavities within the bone. This membrane, like the inner layer of the periosteum, contains osteoblasts, and is important in the formation of new bone tissue.

Bone tissue consists largely of a hard substance called the matrix.