Skeleton and classes of bones in animals

The term skeleton is applied to the framework of hard structures which supports and protects the soft tissues of animals. In the descriptive anatomy of the higher animals it is usually restricted to the bones and cartilages, although the ligaments which bind these together might well be included.

In zoology the term is used in a much more comprehensive sense and includes all the harder supporting and protecting structures.
When the latter are situated externally, they form an exoskeleton, derived from the ectoderm. e.g. the shells and chitinous coverings of many invertebrates, the scales of fishes, the shields of turtles and the feathers, hair and hoofs of the higher vertebrates.
The endoskeleton is embedded in the soft tissues. It is derived chiefly from the mesoderm, but includes the notochord or primitive axial skeleton, which is of entodermal origin.

The skeleton may be divided primarily into three parts:
(1) axial,
(2) appendicular,
(3) splanchnic.

The axial skeleton comprises the vertebral column, ribs, sternum, and skull.
The appendicular skeleton includes the bones of the limbs.
The splanchnic skeleton consists of certain bones developed in the substance of some of the viscera or soft organs, e.g. the os penis of the dog and the os cordis of the ox.

The number of the bones of the skeleton of an animal varies with age, owing to the fusion during growth of skeletal elements which are separate in the foetus or the young subject. Even in adults of the same species numerical variations occur, e. g., the tarsus of the horse may consist of six or seven bones, and the carpus
of seven or eight. In all the domestic mammals the number of coccygeal vertebrae varies considerably.

The bones are commonly divided into four classes according to their shape and function:
(1) Long bones (Ossa longa) are typically of elongated cylindrical form with enlarged extremities. They occur in the limbs, where they act as supporting columns and as levers. The cylindrical part, termed the shaft or body (Corpus), is tubular and incloses the medullary cavity, which contains the medulla or marrow.
(2) Flat bones (Ossa plana) are expanded in two directions. They furnish sufficient area for the attachment of muscles and afford protection to the organs which they cover.
(3) Short bones (Ossa brevia), such as those of the carpus and tarsus, present somewhat similar dimensions in length, breadth, and thickness. Their chief function appears to be that of diffusing concussion. Sesamoid bones, which are developed in the capsules of some joints or in tendons, may be included in this group. They diminish friction or change the direction of tendons.
(4) Irregular bones. This group would include bones of irregular shape, such as the vertebrae and the bones of the cranial base. They are median and unpaired. Their functions are various and not so clearly specialized as those of the preceding classes.

This classification is not entirely satisfactory; some bones, e. g., the ribs are not clearly provided for and others might be variously placed.

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