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 zoölogy 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. Examples of this are 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= (with which we have to deal at present) 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 fœtus 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 vertebræ 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 vertebræ 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.