This, the cubital articulation (Articulatio cubiti), is a ginglymus
formed between the distal extremity of the humerus and the proximal ends
of the bones of the forearm.
[Illustration:
FIG. 143.—LEFT ELBOW JOINT OF HORSE, POSTERIOR VIEW. THE CAPSULE IS
REMOVED. (After Schmaltz, Atlas d. Anat. d. Pferdes.)
]
The =articular surfaces= are: (1) A trochlear surface formed by the
condyles of the humerus and the groove between them; (2) the
corresponding glenoid cavities and ridge on the proximal extremity of
the radius, together with the semilunar notch of the ulna.
The articular surface of the condyles does not extend upon the back of
the extremity, but the groove which receives the semilunar notch of
the ulna extends up into the olecranon fossa. In the fore part of the
groove there is a synovial fossa. The surface on the outer condyle is
smaller than that of the inner one, and is subdivided into two unequal
parts by a shallow furrow. On the lower part of the semilunar notch
and the adjacent part of the ridge on the radius are synovial fossæ.
The =joint capsule= is extremely thin behind, where it forms a pouch in
the olecranon fossa under the anconeus muscle and a pad of fat. In front
it is strengthened by oblique fibers (Ligamentum obliquum or anterior
ligament), and laterally it fuses with the lateral ligaments. Small
pouches of the =synovial membrane= lubricate the origins of the flexors
of the carpus and digit and the small radio-ulnar joints. There are two
lateral ligaments.
The =internal lateral ligament= (Ligamentum collaterale radiale) is
attached above to an eminence on the internal epicondyle of the humerus,
and divides into two parts: the long, superficial part ends on the inner
border of the radius, just below the level of the interosseous space;
the deep, short part is inserted into the internal tuberosity of the
radius. (The superficial part represents the pronator teres muscle,
which is only exceptionally present in the horse.)
The =external lateral ligament= (Ligamentum collaterale ulnare) is short
and strong. It is attached above to a depression on the external
epicondyle of the humerus, and below to the external tuberosity of the
radius, just below the margin of the articular surface.
=Movements.=—This joint is a typical ginglymus, the only movements being
flexion and extension around an axis which passes through the upper
attachments of the lateral ligaments. In the standing position the
articular angle (in front) is about 140° to 150°. The range of movement
is about 55° to 60°. Complete extension is prevented chiefly by the
tension of the lateral ligaments and the biceps muscle. (The axis of
movement is slightly oblique, so that in flexion the forearm is carried
somewhat outward.)
[Illustration: