First to Sixth Thoracic Ventral Rami

The first thoracic ventral ramus divides unequally. A large branch ascends across the neck of the first rib, lateral to the superior intercostal artery, and enters the brachial plexus. The smaller branch is the first intercostal nerve; it runs in the first intercostal space and ends on the front of the chest as the first anterior cutaneous nerve of the thorax. It gives off a lateral cutaneous branch, which pierces the chest wall in front of the serratus anterior and supplies the axillary skin; it may communicate with the intercostobrachial nerve and sometimes joins the medial cutaneous nerve of the arm. The first thoracic ramus often receives a connecting ramus from the second, which ascends in front of the neck of the second rib.

The second to sixth thoracic ventral rami pass forward in their intercostal spaces below the intercostal vessels. At the back of the chest they lie between the pleura and external intercostal membranes, but in most of their course they run between the internal intercostals and the subcostals and innermost intercostals (see Fig. 19.2). Near the sternum, they cross anterior to the internal thoracic vessels and transversus thoracis; pierce the internal intercostals, external intercostal membranes and pectoralis major; and end as the anterior cutaneous nerves of the thorax, which supply the skin on the front of the thorax. The second anterior cutaneous nerve may be connected to the medial supraclavicular nerves of the cervical plexus; twigs from the sixth intercostal nerve supply abdominal skin in the upper part of the infrasternal angle.

Branches

Numerous slender muscular filaments supply the intercostals, serratus posterior superior and transversus thoracis. Anteriorly, some cross the costal cartilages from one intercostal space to another.

Each intercostal nerve gives off a collateral and a lateral cutaneous branch before it reaches the angle of the adjoining ribs. The collateral branch follows the inferior border of its space in the same intermuscular place as the main nerve, which it may rejoin before it is distributed as an additional anterior cutaneous nerve. The lateral cutaneous branch accompanies the main nerve a short way and then pierces the intercostal muscles obliquely. With the exception of the lateral cutaneous branches of the first and second intercostal nerves, each divides into anterior and posterior rami that subsequently pierce the serratus anterior. Anterior branches run forward over the border of the pectoralis major to supply the overlying skin; those of the fifth and sixth also supply twigs to a variable number of upper digitations of external oblique. Posterior branches run backward and supply the skin over the scapula and latissimus dorsi.

The lateral cutaneous branch of the second intercostal nerve is the intercostobrachial nerve (see Fig. 18.9). It crosses the axilla to gain the medial side of the arm and joins a branch of the medial cutaneous nerve of the arm. It then pierces the deep fascia of the arm and supplies the skin of the upper half of the posterior and medial parts of the arm, communicating with the posterior cutaneous branch of the radial nerve. Its size is in inverse proportion to the size of the medial cutaneous nerve. A second intercostobrachial nerve often branches off from the anterior part of the third lateral cutaneous nerve and sends filaments to the axilla and the medial side of the arm.

Seventh to Twelfth Thoracic Ventral Rami

The seventh to twelfth lower thoracic ventral rami continue anteriorly from the intercostal spaces into the abdominal wall (Fig. 19.3). Approaching the anterior ends of their respective spaces, the seventh and eighth nerves curve superomedially across the deep surface of the costal cartilages between the digitations of the transverse abdominis. They reach the deep aspect of the posterior layer of the aponeurosis of internal oblique. Both the seventh and eighth nerves then run through this aponeurosis, pass posterior to the rectus abdominis and supply branches to the upper portion of the muscle. They pass through the muscle near its lateral edge and pierce the anterior rectus sheath to supply the skin of the epigastrium.

Fig. 19.3 Course of the lower intercostal and cutaneous branches of some lumbar nerves. Portions of the muscles of the anterior abdominal wall have been removed, including most of the anterior layer of the rectus sheath and parts of rectus abdominis.

The ninth to eleventh intercostal nerves pass from their intercostal spaces between digitations of the diaphragm and transversus abdominis. They enter the layer between the transversus abdominis and internal oblique. Here, the ninth nerve runs forward almost horizontally, whereas the tenth and eleventh pass inferomedially. At the lateral edge of rectus abdominis, the nerves pierce the posterior layer of the aponeurosis of internal oblique and pass behind the muscle to end, like the seventh and eighth intercostal nerves, with cutaneous branches. The ninth nerve supplies skin above the umbilicus; the tenth supplies skin, which includes the umbilicus; and the eleventh supplies skin below the umbilicus (see Fig. 8.15). The twelfth thoracic nerve (subcostal nerve) connects with the first lumbar ventral ramus (dorsolumbar nerve). It accompanies the subcostal vessels along the inferior border of the twelfth rib, passing behind the lateral arcuate ligament and kidney and anterior to the upper part of the quadratus lumborum. It perforates the transversus abdominis fascia, running deep to the internal oblique, to be distributed like the lower intercostal nerves. It supplies the anterior gluteal skin, reaching down to the greater trochanter.

The seventh to twelfth intercostal nerves supply the intercostal, subcostal and abdominal muscles. The tenth, eleventh and twelfth supply the serratus posterior inferior. All six nerves also provide sensory fibres to the costal parts of the diaphragm and related parietal pleura and peritoneum. Like the upper intercostal nerves, they give off collateral and lateral cutaneous branches before they reach the costal angles. The collateral branch may rejoin its parent nerve; if it does, it leaves again near the lateral border of rectus abdominis. It then runs forward, through the muscle and its anterior sheath near the linea alba to supply the overlying skin. The lateral cutaneous branches pierce the intercostal muscles and external oblique and divide into anterior and posterior branches. These branches supply the skin of the abdomen and back. The anterior branches supply external oblique. The posterior branches pass back to supply the skin over latissimus dorsi. Each lateral cutaneous nerve descends as it pierces external oblique and the superficial fascia and reaches the skin on a level with the anterior and posterior cutaneous nerves of the segment.

Twelfth Thoracic Ventral Ramus (Subcostal Nerve)

The ventral ramus of the twelfth thoracic nerve (subcostal nerve) is larger than the others. It gives a communicating branch to the first lumbar ventral ramus (sometimes termed the dorsolumbar nerve). Like the intercostal nerves, it soon gives off a collateral branch. It accompanies the subcostal vessels along the inferior border of the twelfth rib, passing behind the lateral arcuate ligament and kidney and in front of the upper part of quadratus lumborum. It perforates the aponeurosis of the origin of the transversus abdominis and passes forward between that muscle and internal oblique, to be distributed in the same manner as the lower intercostal nerves. It connects with the iliohypogastric nerve of the lumbar plexus and sends a branch to the pyramidalis. The lateral cutaneous branch of the subcostal nerve pierces the internal and external oblique muscles and supplies the lowest slip of the latter. It descends over the iliac crest approximately 5 cm behind the anterior superior iliac spine (see Fig. 19.3) and is distributed to the anterior gluteal skin; some filaments reach as low as the greater trochanter of the femur.