It is evidently advantageous to employ terms which are as far as possible independent of the position of the body in space and capable of general application, _e. g._, dorsal, ventral, proximal, etc. It is also desirable that the terms internal and external be reserved to indicate relations of depth in cavities or organs, and medial and lateral to designate relations to the median plane. Such terms are coming into more extensive use in human and veterinary anatomy, but the older nomenclature is very firmly established and cannot well be discarded at once and entirely. OSTEOLOGY THE SKELETON 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.