Intercostal Nerves And Arteries
The typical thoracic spinal
nerve is formed by the junction of a dorsal root and a ventral root near the
intervertebral foramen below the vertebra having the same number as the nerve.
The dorsal root is made up of a series of rootlets that emerge from one
segment of the spinal cord between its dorsal and lateral white columns; it
contains the nerve cell bodies of the afferent neurons that enter the spinal
cord through it. This col- lection of nerve cell bodies causes a swelling of
the root, named the dorsal root ganglion. A series of rootlets com-
posed of axons of ventral-born gray cells leaves the same segment of the cord
between the lateral and ventral white columns to form the ventral root of
the spinal nerve.
The dorsal and ventral roots join near the intervertebral foramen to make
up the very short common trunk of the spinal nerve, which divides almost
immediately into the dorsal ramus (posterior primary division) and the ventral
ramus (anterior primary division). The white and gray rami communicantes, which
connect the ganglia of the
sympathetic trunk and the thoracic nerves of the same level, join the ventral
ramus near its origin.
The dorsal ramus of the thoracic nerve, passing posteriorly,
pierces the erector spinae muscle (which it supplies), the trapezius muscle,
and the other superficial muscles of the back (depending on the level) to reach
the superficial fascia. There it divides into a smaller medial branch and a
longer lateral cutaneous branch, which supply the skin.
The ventral ramus of the thoracic nerve is the intercostal nerve
of that particular level (for the twelfth thoracic nerve, the subcostal nerve).
From the seventh to the eleventh thoracic levels, the ventral rami of the
thoracic nerves continue from the intercostal spaces into the anterior
abdominal wall. The intercostal nerve runs forward in the thoracic wall between
the innermost intercostal muscle and the internal intercostal muscle. It lies
inferior to the intercostal vein and intercostal artery and gives off a
collateral branch to the lower part of the space, as do the vein and artery.
The intercostal nerve has a lateral cutaneous branch at the lateral aspect of
the thorax that pierces the overlying intercostal muscles to reach the
subcutaneous tissue. There it divides into an anterior (mammary) and a posterior branch. At the anterior end of
the intercostal space, the intercostal nerve ends by becoming the anterior
cutaneous nerve, which divides into a lateral branch and a shorter and smaller
medial branch.
The aorta, lying on the anterior aspect of the vertebral bodies, gives
off pairs of posterior (aortic) intercostal arteries. The right posterior
intercostal arteries lie on the anterior aspect and the right side of the
vertebral bodies as they travel to reach the intercostal spaces of the right
side. The right and left posterior intercostal arteries course forward in the
upper part of the intercostal spaces between the intercostal vein above and the
intercostal nerve below to anastomose with the anterior intercostal branches of
the internal thoracic and musculophrenic arteries. Collateral branches run in
the inferior parts of the intercostal space.
To reach the pleural cavity from the outside at the anterolateral aspect
of the thorax, a needle would pass through the following layers: skin,
superficial fascia, intercostal muscles and related deep fascial layers, sub-pleural
fascia, and parietal layer of the pleura. If the needle is carefully inserted
near the lower part of the intercostal space (i.e., above the rib margin), one
is reasonaly sure of avoiding the intercostal nerve and vessels.
