Under this head will be described a number of important ligaments which
are connected with the sesamoid bones and form a sort of stay apparatus
or brace.
The =intersesamoidean ligament= (Ligamentum intersesamoideum) not only
fills the space between and unites the sesamoid bones, but also extends
above them, entering into the formation of the articular surface of the
fetlock joint. Other facts in regard to it have been given above.
The =lateral sesamoidean ligaments=, outer and inner (Ligamenta
sesamoidea ulnare et radiale), arise on the abaxial surface of each
sesamoid bone, pass forward, and divide into two branches, one of which
ends in the depression on the distal end of the large metacarpal bone,
the other on the eminence on the proximal end of the first phalanx. They
are partly covered by the branches of the suspensory or superior
sesamoidean ligament.
[Illustration:
FIG. 151.—DEEP DISSECTION OF DISTAL PART OF RIGHT FORE LIMB OF HORSE,
SHOWING JOINTS AND LIGAMENTS, POSTERIOR VIEW.
1, Lateral cartilage; 2, tendon surface of navicular bone; 3, inferior
navicular or interosseous ligament; 4, insertion of deep flexor
tendon. Small arrows point to openings made in capsules of pastern
and coffin joints. (After Schmaltz, Atlas d. Anat. d. Pferdes.)
]
The =suspensory= or =superior sesamoidean ligament= (Musculus
interosseus medius) lies for the greater part in the metacarpal groove,
where it has the form of a wide, thick band. It is attached above to the
upper part of the posterior surface of the large metacarpal bone and to
the distal row of carpal bones. At the lower fourth of the metacarpus it
divides into two diverging branches. Each branch passes to the abaxial
face of the corresponding sesamoid, on which a considerable part, is
attached. The remainder passes obliquely downward and forward to the
anterior surface of the first phalanx, where it joins the extensor
tendon. This ligament possesses considerable elasticity, and is the
highly modified interosseous medius muscle. It consists mainly of
tendinous tissue, but contains a variable amount of striped muscular
tissue, especially in its deep part and in young subjects. Its principal
function is to support the fetlock, _i. e._, to prevent excessive dorsal
flexion of the joint when the weight is put on the limb. The branches
which join the common extensor tendon limit volar flexion of the
interphalangeal joints in certain phases of movement.
The =inferior sesamoidean ligaments= are three in number—superficial,
middle, and deep. The =superficial= or =straight ligament= (Ligamentum
sesamoideum rectum) is a flat band and is somewhat wider above than
below.[34] It is attached above to the bases of the sesamoid bones and
the intersesamoid ligament, below to the complementary fibro-cartilage
of the proximal end of the second phalanx. The =middle ligament= is
triangular, with thick, rounded margins (Ligamenta obliqua) and a thin
central portion.[35] Its base is attached to the sesamoid bones and
intersesamoid ligament, and its deep face to the triangular rough area
on the posterior surface of the first phalanx. The =deep= or =cruciate
ligament= (Ligamenta sesamoidea cruciata) consists of two thin layers of
fibers which arise on the base of the sesamoid bones, cross each other,
and end on the opposite eminence on the proximal end of the first
phalanx.
The =short sesamoidean ligaments= (Ligamenta sesamoidea brevia) are best
seen by opening the joint in front and pushing the sesamoid bones
backward; they are covered by the synovial membrane. Each is a short
band which extends from the anterior part of the base of the sesamoid
bone outward to the posterior margin of the articular surface of the
first phalanx.
The inferior sesamoidean ligaments may be regarded as digital
continuations of the suspensory ligament, the sesamoid bones being
intercalated in this remarkable stay apparatus by which the fetlock is
supported and concussion diminished.