Dermatomes
and Cutaneous Nerves
It may be necessary for a physician to test the integrity of
the spinal cord segments of C3 through T1. It is seen that the dermatomes for
the upper cervical segments C3 to 6 are located along the lateral margin of the
upper limb; the C7 dermatome is situated on the middle finger; and the
dermatomes for C8, T1, and T2 are along the medial margin of the limb. The
nerve fibers from a particular segment of the spinal cord, although they exit
from the cord in a spinal nerve of the same segment, pass to the skin in two or
more different cutaneous nerves.
The skin over the point of the shoulder and halfway down the
lateral surface of the deltoid muscle is supplied by the supraclavicular nerves
(C3 and 4). Pain may be referred to this region as a result of inflammatory
lesions involving the diaphragmatic pleura or peritoneum. The afferent stimuli
reach the spinal cord via the phrenic nerves (C3, 4, and 5). Pleurisy,
peritonitis, subphrenic abscess, or gallbladder disease may therefore be
responsible for shoulder pain.
Superficial
Veins
The veins of the upper limb can be divided into two groups: superficial
and deep. The deep veins comprise the venae comitantes, which accompany all the
large arteries, usually in pairs, and the axillary vein.
The superficial veins of the arm lie in the superficial
fascia.
The cephalic vein ascends in the superficial fascia on the lateral
side of the biceps and, on reaching the infraclavicular fossa, drains into the
axillary vein.
The basilic vein ascends in the superficial fascia on the medial
side of the biceps. Halfway up the arm, it pierces the deep fascia and at the
lower border of the teres major joins the venae comitantes of the brachial
artery to form the axillary vein.
Nerve
Supply of the Veins
Like the arteries, the smooth muscle in the wall of the
veins is innervated by sympathetic postganglionic nerve fibers that provide
vasomotor tone. The origin of these fibers is similar to those of the arteries.
Superficial
Lymph Vessels
The superficial lymph vessels draining the superficial
tissues of the upper arm pass upward to the axilla.
Venipuncture
and Blood Transfusion
The superficial veins are clinically important and are used
for venipuncture, transfusion, and cardiac catheterization. Every clinical
professional, in an emergency, should know where to obtain blood from the arm.
When a patient is in a state of shock, the superficial veins are not always
visible. The cephalic vein lies fairly constantly in the superficial fascia,
immediately posterior to the styloid process of the radius. In the cubital
fossa, the median cubital vein is separated from the underlying brachial artery
by the bicipital aponeurosis. This is important because it protects the artery
from the mistaken introduction into its lumen of irritating drugs that should
have been injected into the vein.
The cephalic vein, in the deltopectoral triangle, frequently
communicates with the external jugular vein by a small vein that crosses in
front of the clavicle. Fracture of the clavicle can result in rupture of this
communicating vein, with the formation of a large hematoma.
Intravenous
Transfusion and Hypovolemic Shock
In extreme hypovolemic shock, excessive venous tone may inhibit
venous blood flow and thus delay the introduction of intravenous blood into the
vascular system.
Anatomy
of Basilic and Cephalic Vein Catheterization
The median basilic or basilic veins are the veins of choice
for central venous catheterization, because from the cubital fossa until the basilic
vein reaches the axillary vein, the basilic vein increases in diameter and is
in direct line with the axillary vein. The valves in the axillary vein may be
troublesome, but abduction of the shoulder joint may permit the catheter to
move past the obstruction. The cephalic vein does not increase in size as it
ascends the arm, and it frequently divides into small branches as it lies
within the deltopectoral triangle. One or more of these branches may ascend
over the clavicle and join the external jugular vein. In its usual method of termination,
the cephalic vein joins the axillary vein at a right angle. It may be difficult
to maneuver the catheter around this angle.
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