DORSAL ASPECT OF THE THORAX
The dorsal aspect of the thorax
is also covered by skin and superficial fascia, with the cutaneous nerves to the
skin of the back ramifying in the latter. These cutaneous nerves are branches
of the posterior primary divisions (dorsal rami) of the thoracic nerves for the
upper six thoracic levels the medial branch and for the lower six the lateral
branch.
The more superficial muscles on the posterior aspect of the thorax belong
to the group connecting the upper extremity to the vertebral column. They are
the trapezius, latissimus dorsi, rhomboideus major, rhomboideus minor, and
levator scapulae.
The trapezius muscle arises from about the medial third of the
superior nuchal line, the external occipital protuberance and the posterior
margin of the ligamentum nuchae, and the spinous processes of the seventh
cervical and all of the thoracic vertebrae and the related supraspinous
ligaments. The lower fibers converge into an aponeurosis that slides over the
triangular area at the medial end of the spine of the scapula and is attached
at the apex of this triangle. The middle group of fibers is inserted on the
medial margin of the acromion and the upper margin of the posterior border of
the spine of the scapula. The upper group of fibers ends on the posterior border
of the lateral third of the clavicle. The trapezius is supplied by the spinal
part of the eleventh cranial nerve and branches from the anterior divisions
(ventral rami) of the third and fourth cervical nerves. When contracting, the
muscle tends to pull the scapula medially while at the same time rotating it,
thus carrying the shoulder superiorly. If the shoulder is fixed, the upper fibers
tilt the head so that the face goes upward toward the opposite side.
The latissimus dorsi muscle has a broad origin—by a small muscular
slip from the outer lip of the iliac crest just lateral to the sacrospinalis
muscle and by an extensive aponeurosis attached to the spinous processes of the
lower six thoracic vertebrae, the lumbar and sacral vertebrae, and the related
supraspinous ligaments. This muscle is inserted into the depth of the
intertubercular groove of the humerus. Its nerve supply comes from the sixth,
seventh, and eighth cervical nerves by way of the thoracodorsal branch of the
brachial plexus. This muscle helps with extension, adduction, and medial rotation at the shoulder joint and helps
to depress the raised arm against resistance.
The rhomboideus major and minor muscles are often difficult to
separate. The rhomboideus major arises from the tips of the spinous processes
and supraspinous ligaments of the second to fifth thoracic vertebrae. Its
insertion is into the vertebral border of the scapula via a tendinous arch
running from the lower angle of the smooth triangle at the root of the spine to
the inferior angle. The rhomboideus
minor muscle arises from the spinous processes of the first thoracic and last
cervical vertebrae and the lower part of the ligamentum nuchae and is inserted
into the vertebral border of the scapula at the base of the triangle, forming
the root of the scapular spine. The rhomboideus muscles are supplied by fibers
from the fifth and sixth cervical nerves by way of the dorsoscapular branch of
the brachial plexus. The rhomboideus major and minor muscles tend to draw the scapula toward the vertebral
column and slightly superiorly, with the lower fibers of the major muscles
helping to rotate the scapula so that the shoulder is depressed.
The levator scapulae muscle originates in four tendinous slips
attached to the transverse processes of the first four cervical vertebrae. Its
insertion is the vertebral border of the scapula from its superior angle to the
smooth triangle at the medial end of the spine scapula. Its nerve supply is
primarily by cervical plexus branches from the ventral rami of the third and
fourth cervical nerves. The levator scapulae, as the name indicates, elevates
the scapula, drawing it medially and rotating it so that the tip of the
shoulder is depressed.
Just deep to the group of muscles connecting the upper extremity to the
vertebral column lie the serratus posterior superior and serratus posterior
inferior muscles.
The serratus posterior superior muscle has an origin via a thin
aponeurosis attached to the lower part of the ligamentum nuchae and to the
spinous processes and related supraspinous ligaments of the seventh cervical
and upper two or three thoracic vertebrae. It is inserted by fleshy digitations
into the upper borders of the second to fifth ribs lateral to their angles. This
muscle helps to increase the size of the thoracic cavity by elevating the ribs.
The serratus posterior inferior muscle arises by means of a thin
aponeurosis from the spinous processes and related supraspinous ligaments of
the last two thoracic vertebrae and the first two or three lumbar vertebrae.
This muscle inserts by fleshy digitations into the lower borders of the last
four ribs, just beyond their angles. It tends to pull the last four ribs
downward and outward. The serratus posterior muscles receive branches of the
ventral rami of the thoracic nerves at the levels at which they are located.
Just deep to the serratus posterior superior muscle lie the thoracic portions of the splenius
cervicis and capitis muscles.
The splenius cervicis muscle has a tendinous origin from the
spinous processes of the third to sixth thoracic vertebrae and wraps around the
deeper muscles to insert by tendinous
fasciculi onto the transverse processes of the upper two or three cervical
vertebrae. The splenius capitis muscle arises from the inferior half of
the ligamentum nuchae and the spinous processes of the seventh cervical and the
first three or four thoracic vertebrae. It is inserted onto the occipital bone
just inferior to the lateral third of the superior nuchal line. The splenius
muscles tend to pull the head and neck backward and laterally and to turn the
face toward the same side. They are
supplied by branches of the posterior primary divisions of the middle and lower
cervical nerves.
The groove lateral to the spinous processes of the thoracic vertebrae is
filled by the sacrospinalis muscle, which is covered by the thoracic part
of the lumbodorsal fascia. Deep to the sacrospinalis muscle lie the short
vertebrocostal and intervertebral muscles; they are not described here.
