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Other Terms: Intercostal nerves, Anterior rami of thoracic nerves, Ventral rami of thoracic nerves, Intercostal nerves set, Nervi intercostales, Rami anteriores nervus thoracicorum, Rami ventrales nervus thoracicorum, Nerfs intercostaux
The intercostal nerves extend along the lower margin of the ribs in the subcostal grooves below the intercostal arteries. They are hidden from view on the medial side of the wide lateral lip. From beneath this lip, they can be drawn by gentle traction upon their lateral cutaneous branches. There are eleven intercostal nerves. They are derived from the anterior divisions of the upper eleven thoracic spinal nerves. The twelfth or last thoracic nerve runs below the last rib, after giving off a communicating branch, the dorsilumbar nerve, to the first lumbar nerve. The distribution of the last thoracic nerve is similar to that of an intercostal nerve. The thoracic nerves are connected by rami communicantes, two communicantes to each nerve. The gangliated trunk of the sympathetic is opposite the heads of the ribs. The first thoracic nerve divides, the greater part joining the brachial plexus of nerves. The smaller, called the first intercostal nerve, continues along the lower border of the first rib to the angle, where it passes between the external and internal intercostal muscles. It differs from the other intercostal nerves in that it gives off no lateral or anterior cutaneous branches. The posterior divisions of the thoracic nerves pass backward between the transverse processes of the vertebrae, divide into a medial and a lateral branch, and supply the muscles and integument of the back. The ten remaining intercostal nerves are divided into an upper and a lower set. The upper set includes the anterior divisions of the second, third, fourth, fifth, and sixth. The lower set includes the anterior divisions of the seventh, eighth, ninth, tenth, and eleventh thoracic nerves. The upper intercostal nerves run outward in the interspaces, at first between the posterior intercostal membrane and the pleura, and then between the external and internal intercostal muscles, until the midaxillary line is reached. Each nerve then gives off a lateral cutaneous branch, which pierces the external intercostal muscle and emerges between the serrations of the serratus anterior muscle. At this point, the main trunk of the nerve enters the substance of the internal intercostal muscle and runs in it until the costal cartilage is reached. Then it passes behind the internal intercostal muscle, lying on the pleura and serrations of the transverses thoracis muscle. After crossing in front of the internal mammary artery it pierces the interspace close to the sternum as the anterior cutaneous nerve. The upper intercostal nerves supply the levatores costarum, serratus posterior superior, external and internal intercostal, subcostals, and transverses thoracis. It also gives off a lateral cutaneous branch and an anterior cutaneous branch. The lateral cutaneous branches vary in size. The second and third are the largest and most important. After piercing the external intercostal muscles, these nerves divide into anterior and posterior branches. The anterior partly supplies the skin over the anterior axillary fold, and gives branches to the mammary gland. The posterior branches supply the skin over the posterior axillary fold. The lateral cutaneous branch of the second intercostal nerve, or the intercostobrachial nerve, communicates with the medial cutaneous nerve of the arm of the brachial plexus. They also communicate with the lateral cutaneous branch of the third intercostal nerve. They are then distributed to the upper portion of the medial and posterior surface of the arm. The anterior cutaneous nerves, or terminal branches of the intercostal nerves, pierce the internal intercostal muscles, the external intercostal fascia, and the pectoralis major muscle in company with the perforating cutaneous branches of the internal mammary artery. They supply the skin over the sternum and over almost the entire pectoral region, and give branches to the mammary gland. The lower intercostal nerves differ from the upper only in the part of their course after they pierce the interspaces. After emerging from between the costal cartilages they pass into the abdominal wall between the transversalis and internal oblique muscles. They pierce the sheath of the rectus muscle to innervate it. They supply the integument over the rectus abdominis muscle. The lateral cutaneous branches are like the upper intercostal nerves. They divide, and supply the integument of the abdomen as far forward as the semilunar line, and the skin of the back over the loins.
Rami anteriores nervus thoracicorum
Rami ventrales nervus thoracicorum