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186 ANATOMY. ;

third they approach eacli other, and are again separated as they near the
clavicle. Close to the hyoid bone the outer margin is in contact with
the orao-hyoid muscle.
Action.—To depress the hyoid bone.
The Sterno-thyroid muscle is shorter and broader than the sterno-
hyoid, and is under that muscle. It arises near its fellow of the oppo-
site side from the posterior surface of the manubrium (first bone of the
sternum), below and nearer the median line than the origin of the
sterno-hyoid nniscle, and inconstantly from the first and. second costal
cartilages. It ascends, and, diverging from its fellow of the opposite
side, is inserted into the oblicpie line on the ala of the thyroid cartilage.
Relations.—Its superficial surface is in contact with the sternum,
sterno-cleido-mastoid, and the sterno-hyoid muscles. The deep surface
rests upon the innominate vein, the lower part of the common carotid
artery, the trachea, and the thyroid gland.
Action.—It depresses the thyroid cartilage and indirectly the floor of
the mouth.
The Thi/ro-h}/oid is a small quadrilateral muscle, its fibres interlacing
with the sterno-thyroid, of which it is, to all appearance, a continuation.
It arises on the oblicpie line on the ala of the thyroid cartilage ; it passes
upward and is inserted into the lower border of the body and great
cornu of the hyoid bone.
Relations.—Its superficial surface is in contact with the sterno-hyoid
and omo-hyoid muscles. Its under surface rests upon the thyroid car-
tilage and thyro-hyoid membrane. The superior laryngeal nerve and
artery pass between the membrane and muscle before entering the
larynx.
Actions.—The thyro-hyoid muscle raises the thyroid cartilage, or,
when that body is fixed, it lowers the hyoid bone.
Nerve.—The muscle is supplied by a branch of the hypoglossal nerve.
The Omo-hi/oid is a long, ribbon-shaped muscle. Math two bellies
united by an intervening tendon ; it extends from the shoulder to the
hyoid bone, crossing the neck diagonally and dividing the anterior and
posterior surgical triangles into four. The muscle arises from the upper
it passes forward
Ijorder of the scapula, near the suprascapular notch ;
and slightly upward, in a flattened narrow fasciculus, across the lower
portion of the neck to the point at which it lies beneath the sterno-
cleido-mastoid muscle, when it becomes tendinous, the tendon being held
down by a loop formed from the deep fascia, which has an attachment
to the (!artilage of the first rib ; it then passes nearly vertically close to
the outer border of the sterno-hyoid muscle, to be inserted into the
lower border of the body of the hyoid bone, in close proximity to, and
outside of, the sterno-hyoid muscle.
Relations.—The superficial surface with the trapezius and sterno-
cleido-mastoid muscles, the deep cervical fascia, the platysma myoides,
and the integument. Its under surface passes over the scaleni, the
brachial plexus, the sheath containing the common carotid artery, the
internal jugular vein and pneumogastric nerve, the noni nerve, and
the sterno-hyoid and sterno-thyroid muscles.
Variations.—The muscle is sometimes divided throughout or in part