Nerves and vessels of Thoracic Wall

Intercostal Nerves

• Each intercostal space contains intercostal muscles, blood vessels, and an intercostal nerve.
• The intercostal nerves are formed by the ventral primary rami of the first eleven thoracic spinal nerves.
• After passing through an intervertebral foramen, each thoracic spinal nerve divides into a ventral primary ramus and a dorsal primary ramus.
• The ventral ramus of the twelfth thoracic spinal nerve continues as the subcostal nerve, which runs below the twelfth rib.

Features of intercostal nerves

• Intercostal nerves represent the ventral primary rami of the thoracic spinal nerves.
• They retain a segmental pattern of distribution and supply defined areas of the body wall without forming a nerve plexus.

Classification

• Intercostal nerves may be classified as typical or atypical according to their distribution.

Typical intercostal nerves

• Typical intercostal nerves remain confined to the thoracic wall.
• The third to sixth intercostal nerves are considered typical intercostal nerves.

Atypical intercostal nerves

• Atypical intercostal nerves supply structures beyond the thoracic wall in addition to their usual thoracic distribution.
• The first intercostal nerve is atypical because most of its fibres join the ventral ramus of the eighth cervical spinal nerve and contribute to the formation of the lower trunk of the brachial plexus.
• The second intercostal nerve gives rise to a large lateral cutaneous branch known as the intercostobrachial nerve.
• The intercostobrachial nerve supplies the skin of the floor of the axilla and the upper part of the medial side of the arm, where it is associated with the medial cutaneous nerve of the arm.
• The seventh to eleventh intercostal nerves extend beyond the thoracic wall to supply the anterior abdominal wall.
• Because of their distribution in both the thorax and abdomen, the seventh to eleventh intercostal nerves are also called thoracoabdominal nerves.

Typical Intercostal Nerves

The third to sixth intercostal nerves are regarded as typical because their distribution is restricted to the intercostal spaces and thoracic wall.

Course and relations

• The course of a typical intercostal nerve can be described in two parts:
  • The segment extending from the intervertebral foramen to the costal groove.
  • The segment travelling within the costal groove.

Course up to the Costal Groove

• After emerging from the intervertebral foramen, the intercostal nerve passes laterally to enter the corresponding intercostal space.
• It initially passes posterior to the sympathetic trunk.
• The nerve then travels within the endothoracic fascia, between the costal pleura and the posterior intercostal membrane.

Course in the Costal Groove

• Within the costal groove, the intercostal nerve runs between the internal intercostal and innermost intercostal muscles.
• It forms part of the intercostal neurovascular bundle.
• Within this bundle, the structures are arranged from superior to inferior as the posterior intercostal vein, posterior intercostal artery, and intercostal nerve.
• This arrangement is commonly remembered as VAN: Vein, Artery, Nerve.
• The intercostal nerve occupies the lowest position in the main neurovascular bundle.
• Anteriorly, the nerve pierces the internal intercostal muscle, anterior intercostal membrane, and pectoralis major muscle.
• It then reaches the anterior thoracic wall and continues as the anterior cutaneous branch.

Branches

• A typical intercostal nerve gives rami communicantes, muscular branches, a collateral branch, a lateral cutaneous branch, and an anterior cutaneous branch.

Rami Communicantes

• Rami communicantes connect the intercostal nerve with the sympathetic trunk.
• They consist of two types:
  • The white ramus communicans contains myelinated preganglionic sympathetic fibres.
  • The gray ramus communicans contains unmyelinated postganglionic sympathetic fibres.

Muscular Branches

• Small muscular branches arise from the intercostal nerve. These branches supply the intercostal muscles and the serratus posterior superior muscle.

Collateral Branch

• The collateral branch usually arises near the angle of the rib. It passes anteriorly along the upper border of the rib immediately below the corresponding intercostal space. It supplies the intercostal muscles, parietal pleura, and periosteum of the ribs.

Lateral Cutaneous Branch

• The lateral cutaneous branch arises near the angle of the rib. It passes through the intercostal muscles near the midaxillary line.
• After reaching the superficial fascia, it divides into anterior and posterior branches. These branches provide sensory innervation to the skin of the lateral thoracic wall.

Anterior Cutaneous Branch

• The anterior cutaneous branch is the terminal branch of a typical intercostal nerve. It emerges close to the lateral border of the sternum after piercing the internal intercostal muscle, anterior intercostal membrane, and pectoralis major muscle.
• It divides into medial and lateral branches. These branches supply the skin over the anterior part of the thoracic wall.

Intercostal Arteries

• The intercostal arteries in each intercostal space are arranged into two groups: anterior intercostal arteries and posterior intercostal arteries.

Posterior Intercostal Arteries (11 pairs)

• There are 11 pairs of posterior intercostal arteries.
• Each intercostal space receives one posterior intercostal artery.
• These arteries supply the greater part of the corresponding intercostal space.

Origin

• The first and second posterior intercostal arteries arise from the superior intercostal artery.
• The superior intercostal artery is a branch of the costocervical trunk, which arises from the subclavian artery.
• The arterial sequence is: Subclavian artery → Costocervical trunk → Superior intercostal artery → First and second posterior intercostal arteries.
• The third to eleventh posterior intercostal arteries arise directly from the descending thoracic aorta.

Course and relations

• The course of a posterior intercostal artery may be described in two parts:
  • The segment extending from its origin to the costal groove.
  • The segment running within the costal groove.

Up to the Costal Groove

• After their origin, the posterior intercostal arteries pass across the anterior aspect of the vertebral bodies before reaching the corresponding intercostal spaces.
• On the right side, the posterior intercostal arteries are longer because the descending thoracic aorta lies on the left side of the vertebral column.
• The right posterior intercostal arteries pass posterior to the esophagus, thoracic duct, azygos vein, and sympathetic trunk.
• On the left side, the posterior intercostal arteries are shorter because of their proximity to the descending thoracic aorta.
• They pass posterior to the azygos venous system and sympathetic trunk.

In the Costal Groove

• Within the costal groove, the posterior intercostal artery forms part of the intercostal neurovascular bundle.
• The neurovascular bundle lies between the internal intercostal and innermost intercostal muscles.
• The structures within the bundle are arranged from superior to inferior as the posterior intercostal vein, posterior intercostal artery, and intercostal nerve.
• This arrangement is commonly represented as VAN: Vein, Artery, Nerve.

Termination

• Each posterior intercostal artery terminates near the costochondral junction by anastomosing with the upper anterior intercostal artery of the corresponding intercostal space.

Branches

• The dorsal branch supplies the muscles and skin of the back. It also gives a spinal branch that contributes to the blood supply of the spinal cord.
• The collateral branch usually arises near the angle of the rib. It runs anteriorly along the upper border of the rib below, accompanied by the collateral branch of the corresponding intercostal nerve. It terminates by anastomosing with the lower anterior intercostal artery.
• The muscular branches supply the muscles of the thoracic wall.
• The lateral cutaneous branch accompanies the lateral cutaneous branch of the intercostal nerve.
• The mammary branches, also called external mammary arteries, arise mainly from the second to fourth posterior intercostal arteries. These branches contribute to the blood supply of the mammary gland.
• The right bronchial artery may arise from the right third posterior intercostal artery.

Anterior Intercostal Arteries

• There are nine pairs of anterior intercostal arteries on each side of the thoracic wall.
• Each of the upper nine intercostal spaces contains two anterior intercostal arteries, one upper and one lower.

Origin

• Anterior intercostal arteries are present only in the upper nine intercostal spaces.
• The anterior intercostal arteries of the first to sixth intercostal spaces arise from the internal thoracic artery.
• The anterior intercostal arteries of the seventh to ninth intercostal spaces arise from the musculophrenic artery.

Termination

• The anterior intercostal arteries terminate by anastomosing with branches of the posterior intercostal arteries near the costochondral junction.
• The upper anterior intercostal artery anastomoses with the corresponding posterior intercostal artery.
• The lower anterior intercostal artery anastomoses with the collateral branch of the corresponding posterior intercostal artery.

Intercostal Veins

• Each intercostal space contains two anterior intercostal veins and one posterior intercostal vein.
• These veins accompany the corresponding intercostal arteries.
• The 10th and 11th intercostal spaces do not contain anterior intercostal veins.

Anterior intercostal veins

• There are nine pairs of anterior intercostal veins on each side of the thoracic wall.
• Anterior intercostal veins are present only in the upper nine intercostal spaces.
• The anterior intercostal veins of the first to sixth intercostal spaces drain into the internal thoracic vein.
• The anterior intercostal veins of the seventh to ninth intercostal spaces drain into the musculophrenic vein.

Posterior intercostal veins

• There are eleven posterior intercostal veins on each side, with one vein present in each intercostal space.
• Each posterior intercostal vein accompanies the corresponding posterior intercostal artery.
• The pattern of drainage of the posterior intercostal veins differs on the right and left sides.
• Their termination depends on their level and their relationship with the azygos and hemiazygos venous systems.

Termination of Posterior Intercostal Veins

• The drainage pattern of the posterior intercostal veins differs on the right and left sides.

First Posterior Intercostal Vein

• The right first posterior intercostal vein drains into the right brachiocephalic vein. The left first posterior intercostal vein drains into the left brachiocephalic vein.

Second to Fourth Posterior Intercostal Veins

• On the right side, the second, third, and fourth posterior intercostal veins unite to form the right superior intercostal vein. The right superior intercostal vein drains into the azygos vein.
• On the left side, the second, third, and fourth posterior intercostal veins unite to form the left superior intercostal vein. The left superior intercostal vein drains into the left brachiocephalic vein.

Fifth to Eighth Posterior Intercostal Veins

• On the right side, the fifth to eighth posterior intercostal veins drain into the azygos vein.
• On the left side, the fifth to eighth posterior intercostal veins drain into the accessory hemiazygos vein.

Ninth to Eleventh Posterior Intercostal Veins

• On the right side, the ninth to eleventh posterior intercostal veins drain into the azygos vein.
• On the left side, the ninth to eleventh posterior intercostal veins drain into the hemiazygos vein.

Lymphatic Drainage of Thoracic Wall

Lymph from the thoracic wall drains mainly into the anterior intercostal nodes and posterior intercostal nodes.

Anterior intercostal nodes

• The anterior intercostal nodes are also known as parasternal nodes or internal mammary nodes. These lymph nodes are situated along the internal thoracic vessels.
• They receive lymph from the anterior parts of the intercostal spaces. Their efferent lymphatics drain toward the tracheobronchial and brachiocephalic lymph nodes.

Posterior intercostal nodes

• The posterior intercostal nodes are located near the heads and necks of the ribs.
• They receive lymph from the posterior parts of the intercostal spaces.
• Lymph from the upper intercostal spaces drains into the thoracic duct on the left side and the right lymphatic duct on the right side.
• Lymph from the lower four intercostal spaces drains toward the cisterna chyli.

Internal Thoracic Artery

• The internal thoracic artery (ITA) is also known as the internal mammary artery.
• A right and a left internal thoracic artery are present.

Location

• Each internal thoracic artery lies deep to the anterior thoracic wall on either side of the sternum.

Origin

• The internal thoracic artery arises from the first part of the subclavian artery.
• Its origin is located opposite the origin of the thyrocervical trunk.

Course

• The artery descends vertically behind the medial end of the clavicle and the upper six costal cartilages.
• It runs approximately 1 cm lateral to the lateral border of the sternum.

Termination

• The internal thoracic artery terminates in the sixth intercostal space.
• It divides into two terminal branches:

• The superior epigastric artery.
• The musculophrenic artery.

Relations

• The relations of the internal thoracic artery may be described in two parts:
  • Above the first costal cartilage.
  • Below the first costal cartilage.

Above the First Costal Cartilage

• Anteriorly, the artery is related to the medial end of the clavicle, internal jugular vein, brachiocephalic veins, and first costal cartilage.
• Posteriorly, it is related to the cervical pleura.

Below the First Costal Cartilage

• Anteriorly, the artery is related to the upper six costal cartilages, external intercostal membranes, internal intercostal muscles, and first six intercostal nerves.
• Posteriorly, it is related to the endothoracic fascia and pleura.
• Below the second or third costal cartilage, the sternocostalis muscle intervenes between the artery and the pleura.

Branches

• The internal thoracic artery gives the pericardiophrenic, mediastinal, anterior intercostal, and perforating branches.

Pericardiophrenic Artery

• The pericardiophrenic artery arises near the root of the neck. It descends through the thorax in association with the phrenic nerve. It supplies the pericardium, pleura, and diaphragm.

Mediastinal Branches

• The mediastinal branches are small vessels arising from the internal thoracic artery. They supply the thymus and mediastinal fat.

Anterior Intercostal Arteries

• The internal thoracic artery gives rise to two anterior intercostal arteries in each of the upper six intercostal spaces.

Perforating Branches

• The perforating branches pass through the internal intercostal muscles. They accompany the anterior cutaneous branches of the intercostal nerves.
• In females, the perforating branches of the second, third, and fourth intercostal spaces are relatively large. These branches contribute to the arterial supply of the breast.

Superior Epigastric Artery

• The superior epigastric artery is one of the terminal branches of the internal thoracic artery. It descends posterior to the seventh costal cartilage.
• The artery enters the rectus sheath by passing between the sternal and costal slips of the diaphragm. Within the rectus sheath, it anastomoses with the inferior epigastric artery.

Musculophrenic Artery

• The musculophrenic artery is the other terminal branch of the internal thoracic artery. It descends obliquely behind the seventh, eighth, and ninth costal cartilages. It gives rise to two anterior intercostal arteries in each of the seventh, eighth, and ninth intercostal spaces.
• The artery supplies the intercostal muscles, diaphragm, and anterior abdominal wall.
• It pierces the diaphragm near the ninth costal cartilage.

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