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AREOLAR TISSUE, TENDONS, AND MUSCLES. 189

known to arise entirely from the styloid process. In rare instances the
muscle has been monogastric, in which case it is inserted into the mid-
dle of the lower jaw. Slips may pass from the anterior belly to the
hyoid bone. The tendon does not always pass through the stylo-
hyoid muscle.
The Ilento-hyoid (Macalister) is an occasional slip found passing from
the body of the hyoid bone to the chin. It is sometimes composed of
parallel bands, and Macalister suggests that it may be a differentiated
portion of the platysma myoides.
Nerves.—The posterior belly of the muscle is supplied by the facial
(seventh), and the anterior by the mylo-hyoid branch of the inferior
dental nerve.
Actions.—The digastric muscles act in antagonism to the muscles of
mastication by assisting in depressing the inferior maxilla. AVhen the
lower jaw is firmly fixed by the masticatory muscles, the digastric
assists in elevating the hyoid bone.
The Stylo-hyokl is a small slender muscle situated along the upper
border of the posterior belly of the digastric. It arises by a narrow
tendon from the upper half of the outer surtace of the styloid process of
the temporal bone. It passes downward, forward, and inward, to be
inserted into the hyoid bone at the junction of the great cornu with the
body. It is usually divided into two portions near its insertion for the
transmission of the digastric muscle.
Relations.—These are almost identical with those of the posterior belly
of the digastric muscle.
Variations.—The variations of the muscle are numerous : amongst
them mav be noted cleavage throughout its whole course, forming two
muscles, in some instances three. It is occasionally placed on the
inner side of the external carotid artery ; the insertion is sometimes
partiallv or wholly in the tendon of the digastric muscle. It may be
fused with the omo-hyoid, thyro-hyoid, or mylo-hyoid muscles at the
hyoid bone. It may send slips to the lower jaw. Its place of origin
varies ; sometimes there is an extra slip given off' from the styloid pro-
cess and inserted into the small cornu, and accompanying or taking the
place of the stylo-hyoid ligament.
Nerves.—The muscle is supplied by the facial nerve.
Action.—To elevate and draw backward the hyoid bone.
The Stylo-hyoid Liejament.—This being so intimately associated with
this group of muscles, it ^yill receive next consideration. The ligament
is a thin fibrous cord developed from the deep fascia attached to the
lower portion of the styloid process, passing downward, forward, and
inward, to be inserted into the lesser cornu of the hyoid bone. Some-
times this ligament is of a cartilaginous nature, and even ossifies. In
many animals it is naturally osseous, and is named the epihyal bone.
The Myh-hyoid is a triangular flat muscle placed between the inferior
maxillary and hyoid bones, and with its fellow of the opposite side
forms the muscular floor of the mouth (diaphragma oris, Meyer). _ It
arises from the mylo-hyoid (internal oblique) ridge of the lower jaw,
extending from about the third molar tooth to the sym]ihysis. At its
insertion" it is divided into twa- portions, a posterior and an anterior.