The proximal phalanx (first phalanx) is composed by a proximal extremity (base), a shaft (body) and a distal extremity (head).
On both extremities are articular facets and prominences for ligamentous attachments.
In horses, the proximal phalange is sometimes called os compedale. It is shaped like a dorsopalmary compressed cylinder. The base is wider than the head. On posterior part of the base of the proximal phalanx are the medial and lateral palmar eminences that are prominent structures where respectively the medial part and the lateral part of the collateral ligaments of the fetlock joint and the medial and lateral short sesamoid ligaments insert. The articular surface of the base is subdivided into a larger medial cavity and a smaller lateral one by a sagittal groove. The palmar surface of the body shows a rough triangular area called the trigonum phalangis proximalis which is bounded by bony ridges.