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AREOLAR TISSUE, TENDONS, ASD 3IUSCLES. 185

base of the skull, terminating in a thin aponeurosis ^vllere it is attached
to the outer two-thirds of" the line.
delations.—Its middle three-fifths are covered superficially by the
platysma myoides, the remainder by the integument, and it is crossed
by the external jugular vein and the superficial branches of the cervical
plexus. Its deep surface passes over the sterno-hyoid, sterno-thvroid,
omo-hyoid, the posterior belly of the digastric, levator anguli scapulae,
the splenius, and the scaleni muscles ; also the cervical plexus, the
occipital arteiy, and a part of the parotid gland.
The common carotid artery, the internal jugular vein, and the pneu-
mogastric nerve enclosed in their sheath, clescendens noni, and com-
municaris noni nerves, pass under its anterior border, and the spinal
accessory nerve pierces its upper third.
Nerves.—The muscle is supplied by the deep cervical plexus and the
spinal accessory nerves.
Variations.—The muscle is sometimes divided longitudinally into
two portions, called the sterno-mastoid and cleido-mastoid ; they are
not infrequently described as separate muscles. Part of the muscle is
sometimes attached to the lower jaw ; this condition is normal in the
bone. Besides this, it has many other varieties (see Quain and Allen).
Action.—AVhen both muscles are acting together, the head is brought
forward, as in nodding ; when extreme action is brought about, the
head is drawn upon the neck and the neck upon the chest. When
either muscle acts singly, especially when combined with the splenius,
the head is drawn toward the shoulder of the same side, the face being
rotated to^^ard the opposite side. In the condition known as Avry neck
or torticollis, the muscle on one side is rigidly contracted, or the opposite
muscle is paralyzed. When the head is fixed, these muscles become
accessory muscles of respiration by assisting in the elevation of the
thorax; it also serves to fix the clavicle, and in animals where the
clavicle is lacking the muscles assist in the elevation of the arms, as
their fibres are continued into the clavicular portion of the pectoralis
major and the deltoid.
The Infra-hyoid 31uscles.—The depressors of the hyoid bone and the
larynx are the sterno-hyoid, the sterno-thyroid, the thyro-hyoid, and
the omo-hyoid.
The Sterno-hyoid is a thin, flat band of longitudinal fibres, arising
inconstantly from the upper and posterior portion of the sternum and
the posterior sterno-clavicular ligament ; from that ligament and the
clavicle, or from the clavicle alone ; and occasionally it partially arises
from the cartilage of the first rib. The fibres pass upward, and are
inserted into the body of the hyoid bone near the inner side of the omo-
hyoid muscle.
Relations.—Its superficial surface below is covered by the sternum
and sternal end of the clavicle, and by the sterno-cleido-mastoid and
the platysma myoides muscles above. Its deep surface passes over the
sterno-thyroid, crico-thyroid, and thyro-hyoid muscles, and in part the
thyroid gland, the superior thyroid vessel, and the crico-thyroid and
thyro-hyoid membranes. The mesial borders of the two muscles vary in
their proximity : at the upper third there is a slight interval ; in the middle