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190 ANATOMY.

The Posterior Portion consists of those fibres wliich give the muscle
its name. They pass downward, inward, and backward, to be inserted
into the body of tlie hyoid bone.
The Anterior Portion passes in a more oblique direction, and is not
inserted into the hyoid bone, but into an indistinct intermuscular raphe
which extends from the symphysis of the jaw to the centre of the hyoid
bone ; the muscular fibres are the longest near the hyoid bone, and
gradually grow shorter as the symphysis of the jaw" is approached.
Relations.—The mylo-hyoid muscle forms the floor of the mouth,
and at the same time part of the roof of the neck, thus giving it an
oral or superior and a cervical or inferior surface. The cervical surface
is in relation with the submaxillary muco-salivary gland, the anterior
belly of the digastric muscle, the facial artery and its submental branches,
and the mylo-hyoid vessels and nerves. The oral surface is in relation
with the genio-hyoid, genio-glossus, parts of the hyo-glossus and stylo-
glossus muscles; also the lingual branch of the fifth and twelfth nerves,
the sublingual gland, and the mucous membrane of the alveolo-lingual
groove. Its posterior border is free, a part of the submaxillary muco-
salivary gland curving around it to the upper surface, the duct of the
gland (duct of Wharton) passing along the upper surface of the muscle.
Variations.—Sometimes the raphe is absent ; in such cases the fibres
of each muscle interlace. It may be fused with the anterior belly of
the digastric muscle, or it may be entirely lacking and be substituted
by the digastric. Slips are sometimes received from some of the other
hyoid muscles. Occasionally the anterior portion of the muscle is defi-
cient, its origin extending no farther than the cuspid teetli : this muscle
is sometimes perfi)rated and dissected by the lobules and duct of the
submaxillary muco-salivary gland.
Nerves.—The mylo-hyoid nerve, a branch of the inferior maxillary.
Action.—The mylo-hyoid muscle draws the hyoid bone forward and
upward, and slightly assists in opening the mouth.
The Genio-hyoid is a narrow muscle extending from the symphysis
of the chin to the hyoid bone. It arises from the inferior genial
tubercle (mental spine) on the inner side of the inferior maxillary bone,
and passes downward and backward to be inserted into the anterior
portion of the hyoid bone.
Relations.—Below with the mylo-hyoid muscle, above ^vitli the genio-
glossus and with its fellow on the proximal border.
Variations.—The genio-hyoid may separate into two muscles, or it
may be united with the muscle of the opposite side. Slight variations
may be found between its origin and insertion.
Nerve.—The genio-hyoid is supplied by a branch of the hypoglossal
nerve.
Action.—Same as the mylo-hyoid—to elevate and draw forward the
hyoid bone and to depress the lower jaw.
The Genio-f/lossus (often called genio-hyo-glossus, from its sup-
posed insertion on the body of the hyoid bone) is a thin, flat, radiat-
ing muscle, placed vertically on each side of the median line in front
of the tongue. It arises by a short tendon from the superior genial
tubercle (mental spine) on the inner -aspect of the inferior maxillary