BONES

CLASSIFICATION OF BONES

The classification of bones is based on three principles: the shape and structure of a bone, its development and function (Figs. 33-34). Bones are divided into long (tubular), short (spongy), irregular (mixed) and pneumatic (Fig. 32).

STRUCTURE OF THE BONES

Bones have a very specific place in the human organism.

Bones consist of different kinds of tissues, mainly, however, of ostial tissue, which is a variety of connective tissue (Figs. 35-38).

Bones have a complex structure and chemical composition. In living organisms bones are 50 pre-

sent water, 28.5 percent organic substances and 21.85 percent inorganic material.

Between them compact and cellulous (spongy) bones are constructed. Compact bone forms dia-physis of tubular bones and covers epiphyses on the outside with thin lamelle (Fig. 38).

Compact bone tissue is perforated by thin canals which contain blood vessels and nerves. Some canals run parallel with the surface of the bone (central or canals) (Fig. 36).

Spongy (cancellous, trabecular) bone has the appearance of a sponge and formed by bone tra-beculae with spaces between them (Fig. 37).

The external surface of bones, excepting their articular surface, is covered by a connective tissue membrane called periosteun (Fig. 35).

OSTEOLOGY

(BONES, OR SKELETAL SYSTEM) Bones of the trunk

Development of the bones of the trunk. The axial skeleton, vertebrae and ribs are derived from paraxial mesenchyme. The regularity of somite formation provides criteria for staging embryos. The somite gave rise to segmental portions of the dermis of the skin, bone and muscle. Sclerotomal populations are organized from the ventral half of the epithelial somite. The original sclerotomes are co-extensive with the individual metameric body segments. Each vertebra is formed by the combination of the caudal half of one bilateral pair of sclerotomes with the cranial half of the next caudal pair of sclerotomes. Their fusion surrounds the notochord. The midline centrum encloses the notochord and lies ventral to the neural tube. The neural arch consists of paired bilateral pedicles (ventrolaterally) and laminae (dorsolaterally). The segmental nature of the vertebrae is promoted by the notochord, which induces the ventral elements of the vertebrae and represses dorsal structures. Dorsal segmentation is influenced by the spinal ganglia.