The five muscles of this group are either reduced to vestiges or
modified greatly in structure.
1, 2. =Lumbricales= (internus et externus).—These are two small muscles
which lie on either side of the flexor tendons above the fetlock. They
arise from the deep flexor tendon, and are lost in the fibrous tissue
which lies under the nodule of horn or ergot at the fetlock (Fig. 447).
Their action is inappreciable. The size of these muscles is subject to
much variation. Often very little muscular tissue can be found, but the
small tendon is constantly present.
_Blood-supply._—Metacarpal arteries.
_Nerve-supply._—Median and ulnar nerves.
3, 4, 5. =Interossei.=—These are three in number in the horse, and are
situated chiefly in the metacarpal groove. Two, the =internus= and
=externus=, are very small muscles, each of which arises from the
corresponding small metacarpal bone near its proximal extremity, and is
provided with a delicate tendon which is usually lost in the fascia at
the fetlock (Fig. 150). They have no appreciable action. Their blood-
and nerve-supply is the same as that of the preceding muscles.
The =interosseus medius= is so much modified that it is usually termed
the =suspensory=, or =superior sesamoidean ligament=. It contains little
muscular tissue, being transformed very largely into a strong tendinous
band, bifurcate below, and having for its chief function the supporting
of the fetlock. It has been described, in deference to common usage,
with the ligaments.