The =ulna= of the horse is a reduced long bone situated behind the
radius, with which it is partially fused in the adult.
The =shaft= (Corpus ulnæ) is three-sided and tapers to a point below.
The =anterior surface= (Facies dorsalis) is applied to the posterior
surface of the radius, and below the interosseous space the two bones
are fused in the adult. The surface which enters into the formation of
the space is smooth and usually presents a small nutrient foramen,
directed upward. Above the space it is rough and is attached to the
radius by an interosseous ligament which is usually permanent. The
=internal surface= (Facies medialis) is smooth and slightly concave. The
=external surface= (Facies lateralis) is flattened. The =internal= and
=external borders= are thin and sharp, except at the interosseous space.
The =posterior border= is slightly concave in its length and is rounded.
The lower end is pointed and is usually a little below the middle of the
radius. It is commonly continued by a fibrous cord to the distal
external tuberosity of the radius, but this band may be replaced in part
or entirely by bone.
The =proximal extremity= is the major part of the bone. It projects
upward and somewhat backward behind the lower end of the humerus, and
forms a lever arm for the extensor muscles of the elbow. The =internal
surface= is concave and smooth. The =external surface= is convex and is
roughened above. The =anterior border= bears on its middle a pointed
projection, the =processus anconæus= or “beak,” which overhangs the
=semilunar notch= or =sigmoid cavity= (Incisura semilunaris). The latter
is triangular in outline, concave from above downward, and articulates
with the humerus; in the middle of its lower part is an extensive
synovial fossa. Just below the notch are two convex =facets= which
articulate with those on the posterior aspect of the proximal end of the
radius. The =posterior border= is nearly straight, and is thick and
rounded. The free end or summit is a rough tuberosity, the =olecranon=,
which gives attachment to the triceps brachii and other muscles.
The primitive =distal extremity= has, as previously stated, fused with
the radius.
=Development.=—The ulna ossifies from three centers, of which one is for
the main part of the bone, one for the olecranon, and one for the distal
end. The cartilaginous embryonic ulna extends the entire length of the
forearm. The lower part of the shaft is usually reduced to a small
fibrous band or may disappear entirely; in some cases a variable remnant
of it ossifies. The distal extremity fuses early with the radius. The
olecranon unites with the rest of the bone at three to three and a half
years. A medullary canal appears to occur constantly in the
adult—contrary to the statements of some authors.