OSTEOQLOGY 
THE SKELETON 
The term skeleton is applied to the framework of hard structures which sup- 
ports 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 zoélogy 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 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) appen- 
dicular; (3) splanchnic. 
The axial skeleton comprises the vertebral column, ribs, sternum, and skull. 
The appendicular skeleton includes the bones of the limbs. 
The splanchnic or visceral 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, é. 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 coceygeal 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 func- 
tion appears to be that of diffusing concussion. Sesamoid bones, which are de- 
veloped 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 (Ossa irregularia). This group would include bones of 
irregular shape, such as the vertebrxe 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. 
1 This classification is not entirely satisfactory; some bones, e. g., the ribs, are not clearly 
provided for, and others might be variously placed. 
20 
