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AREOLAR TISSUE, TESDOXS, AXB MUSCLES. 183

The external pterygoid, having its fibres directed backward, and
nearly all of them horizontally, di-aws the condyle forward and brings
the interarticular fibro-cartilage upon the eniinentia artieularis ; when
the muscles of both sides act in unison, they cause the lower jaw to pro-
ject. Their action is usually alternate, causing a sort of oscillating or
grinding motion of the riiolar teeth. The superficial portion of the mas-
seter acts in conjunction with the external pterygoid muscle in drawing
the jaw forward, while the posterior fibres of the temporal antagonize it,
drawing the jaw backward, thus acting for the trituration of the food.

THE MUSCLES OP THE NECK.
The Platysma Myoides lies immediately below the skin on the side
of the neck. It is a broad^ thin^ quadrangular, pale-colorecl sheet
of muscular fibres, superficial to the deep cervical fascia, extending
over the front and sides of the neck and the loMcr portion of the
face. It arises by thin bands from the subcutaneous connective tissue
over the deltoid, pectoral, and trapezius muscles : the fibres are directed
obliquely upward and forward over the clavicle and acromion process
to the side of the neck, gradually converging and approaching its fellow
of the opposite side, the most anterior fibres crossing over and inter-
lacing witli each other in front of and below the chin. The greater
number of fibres are inserted in the outer surface of the lower jaw,
below the external oblique line anterior to the masseter muscle ; others
pass upward to the lower lip and angle of the mouth ; while others are
lost in the muscles of expression and the areolar connective tissue of
the face.
Variations.—The platysma myoides of man is the rudiment of the
panniclilus carnosus, or great subcutaneous muscle, of quadrupeds; this
may explain its many variations in the human subject. Sometimes the
fibres extend upon the face and downward on the neck, shoulder, and
l)reast farther than usual, occasionally having attachment to the clavicle,
and also give off slips which pass from one muscle of expression to
another. The upper part of the muscle is occasionally joined by a slip
from the occipital bone or the mastoid process of the temporal ; more
rarely it is absent on one or both sides.
Vessels.—The numerous superficial branchings of the region.
Xerves,—The platysma myoides receives its principal nerves of motion
from the descending branches of the facial ; it is also influenced by
some of the spinal nerves.
Relations.—Above with the skin, to which it is closely united, espe-
cially in its lower portion ; by its internal surface with the pectoralis
major, deltoid, trapezius, and the clavicle ; in the neck with the deep
cervical fascia, between which and the muscle passes the superficial
cervical "plexus of nerves, the external jugular vein and its tributaries,
and the anterior jugular vein. In the supra-hyoid region the facial
artery lies underneath it, separated by the deep cervical fascia; all
the superficial cervical lymphatics, sterno-cleido-raastoideus, omo-hyoid,
sterno-hyoid, and diagastric muscles are under it.
Actions.—The platysma elevates the skin of the breast and shoulder,