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188 ANATOMY.

and liolds it in connection with the body and the great cornu of the
hyoid bone. The anterior belli/ is shorter and broader than the pos-
terior. It commences at the intermuscular tendon, passes upward and
forward, and is inserted into the rough depression on the internal lower
border of the inferior maxillary bone near the symphysis : its fibres
sometimes decussate with those of its fellow-muscle on the opposite side.
Stretching between the intermediate tendons and sheaths of the ante-
rior portions of these muscles of each side is a broad layer of fascia known
as the supra-hijokl aponeurosis, which is also attached to the great cornu
of the hyoid bone and the anterior bellies of each muscle ; this gives a
firm support for the other muscles in the supra-hyoid space. This
aponeurosis corresponds to a layer of muscular fibres belonging to
the digastric muscle of some of the lower animals.
The digastric mu;?cle subdivides the anterior superior surgical triangle
of the neck into the submaxillary and superior carotid triangles.
The Subma.villari/ Triangle is inverted, the base being above, and is
bounded by the lower border of the inferior maxillary bone and a line
to the mastoid process ; the other or lower sides are formed by the two
bellies of the digastric muscle, its tendon being held down by a loop of
fibrous tissue which forms the inferior angle. The outer or superficial
surface of this triangle is covered by a firm layer of the deep fascia
attached below to the tendon and to the bellies of the muscle, above to
the body of the inferior maxilla and to the fascia which extends over
the parotid gland. The inner surface is bounded by a deep layer of the
same fliscia attaclied below to the tendon and muscle, while above it is
lost in the sheaths of the muscles of the floor of the mouth. Surgically
considered, it is continuous over those muscles, and is attached to the
lower border of the mylo-hyoid ridge of the lower jaw. In the tume-
faction of the submaxillary gland the enlargement exhibits a triangular
form on account of the shape of this envelope.
The fiuperior Carotid Triangle is bounded above by the posterior
belly of the digastric, below by the omo-hyoid, and behind l)y the
sterno-cleido-mastoid muscle, its apex presenting anteriorly at the loop
of the digastric and insertion of the stylo-hyoid muscle. The import-
ance of this triangle to the surgeon is due to the fact that in it are
found the points for ligation of many arteries.
fielatio}ts.—The anterior belly of the digastric muscle is more super-
ficial than the jiosterior its outer surface being covered by the platysma
;
myoides and tlie deep cervical fliscia, its deep surface rests uj)on the
mylo-hyoid muscle. Its posterior belly is deeply covered by the mas-
toid process of the temporal bone, the sterno-cleiclo-mastoid, and part of
the stylo-hyoid muscles, a lobule of the parotid gland, and part of the
submaxillary gland. Its deep surface is in relation with the transverse
process of the atlas, the internal jugular vein, the internal carotid artery,
and the origins of the facial and lingual arteries. The hypoglossal
nerve lies a little below the tendon.
Variations.—This muscle has many variations, and, like the omo-
hyoid, the entire muscle may be divided through one or both bellies.
The. posterior belly may receive an accessory slip from the styloid
it has been
process, the angle of the jaw bone, or the splenius muscle ;