1. =Mastoido-humeralis= (M. brachiocephalicus; levator humeri).—This
muscle extends along the side of the neck from the head to the arm. It
is incompletely divisible into two portions.
_Origin._—(1) The mastoid process of the petrous temporal bone and the
occipital crest; (2) the wing of the atlas and the transverse processes
of the second, third, and fourth cervical vertebræ.
_Insertion._—The deltoid tuberosity and the curved rough line which
extends from this to the distal extremity of the humerus.
_Action._—When the head and neck are fixed, to draw the limb forward,
extending the shoulder joint. When the limb is fixed, to extend the head
and neck, if the muscles act together; acting separately, to incline the
head and neck to the same side.
_Structure._—As already mentioned, the muscle is capable of incomplete
division into two parts, the line of division being indicated by the
emergence of superficial branches of the ventral divisions of the
cervical nerves. The mastoid portion (M. cleido-mastoideus) partly
overlaps the other portion (M. cleido-transversarius), which lies dorsal
to it. The former is attached to the mastoid process and the occipital
bone by a broad tendon which fuses with that of the splenius and
trachelo-mastoideus; it is also attached to the tendon of insertion of
the sterno-cephalicus by aponeurosis. The dorsal portion is attached to
the transverse processes by four fleshy digitations. The belly of the
muscle is adherent superficially to the cervical fascia and the
panniculus, and deeply to the subscapulo-hyoideus. In front of the
shoulder its deep face is marked by a tendinous intersection of variable
development.[56] Here the muscle becomes wider, covers the shoulder
joint, passes between the brachialis and biceps, and is inserted by
means of a wide tendon which it shares with the superficial pectoral
muscle.
_Relations._—Superficially, the skin, cervical fascia, the parotid
gland, the panniculus, brachialis, and branches of the cervical nerves;
deeply, the splenius, trachelo-mastoideus, rectus capitis anterior
major, omo-hyoideus, serratus magnus, anterior deep pectoral and biceps
muscles, the inferior cervical artery, the prescapular lymph glands, and
branches of the cervical nerves. The ventral edge of the muscle forms
the dorsal boundary of the jugular furrow. The dorsal border may be in
contact with the cervical trapezius, or separated from it by a variable
interval.
_Blood-supply._—Inferior cervical, carotid, and vertebral arteries.
_Nerve-supply._—Spinal accessory and cervical nerves.
The =pectoral fascia= is a thin membrane covering the surface of the
pectoral muscles, to which it is, for the most part, pretty intimately
attached. It detaches a layer which passes between the superficial and
deep pectorals. At the posterior edge of the triceps another layer is
given off, which passes on the outer surface of this muscle to blend
with the scapular fascia; the deeper layer becomes continuous with the
subscapular and cervical fasciæ.
The =pectoral muscles= form a large fleshy mass which occupies the space
between the ventral part of the chest-wall and the shoulder and arm.
They are clearly divisible into a superficial and a deep layer. The
superficial layer may be subdivided into two portions by careful
dissection; the deep layer is clearly made up of two muscles.