Anatomically, the body of the ulna is that portion of the bone distal to the level of the proximal radial physis. The olecranon is the bone proximal to the physis and is the primary site of attachment of the triceps tendon. In addition to its function as a major component of the humeroulnar joint, the olecranon functions as the lever arm, serving to extend the elbow when the triceps muscle contracts. When the elbow extends, the carpus also extends. When both the elbow and carpus are in the fully extended position, the stay apparatus is engaged, allowing the horse to bear full weight on the limb without muscular effort.
The separate center of ossification in the developing olecranon (red arrow) is an apophysis. It serves as the point of attachment for the triceps tendon and is subjected to tensile forces. Growth at the adjacent physis only affects the conformation of the olecranon and does have a major impact on the length or conformation of the limb. This represents a distinct difference from the separate centers of ossification found at the ends of most long bones such as the proximal aspect of the radius (yellow arrow). These epiphyses are components of the adjacent articulation and subjected primarily to compressive forces. Growth from the physes adjacent to these epiphyses are responsible for the length and conformation of the bone and subsequently also of the limb. It is the later group of epiphyses that the classification system defined by Salter and Harris is intended. Fractures of the olecranon are classified by a different scheme.