The<unk>  are the anterior divisions (rami anteriores; ventral divisions) of the thoracic spinal<unk>  from T1 to T11.
Each<unk> is connected with the adjoining ganglion of the sympathetic trunk by a gray and a white ramus communicans. The<unk>  are distributed chiefly to the thoracic pleura and abdominal peritoneum, and differ from the anterior divisions of the other spinal<unk> , in that each pursues an independent course without plexus formation.
The first two<unk>  supply fibers to the upper limb in addition to their thoracic branches; the next four are limited in their distribution to the parietes of the thorax; the lower five supply the parietes of the thorax and abdomen. The 7th<unk> terminates at the xyphoid process, at the lower end of the sternum. The 10th<unk> terminates at the umbilicus. The twelfth (subcostal) thoracic is distributed to the abdominal wall and groin.
The anterior division of the first thoracic<unk> divides into two branches: one, the larger, leaves the thorax in front of the neck of the first rib, and enters the brachial plexus; the other and smaller branch, the first<unk>  runs along the first<unk>  space, and ends on the front of the chest as the first anterior cutaneous branch of the thorax.
Occasionally this anterior cutaneous branch is missing.
The first<unk> rarely gives off a lateral cutaneous branch; but sometimes sends a small branch to communicate with the intercostobrachial.
From the second thoracic<unk> it frequently receives a connecting twig, which ascends over the neck of the second rib.
The anterior divisions of the second, third, fourth, fifth, and sixth thoracic<unk> , and the small branch from the first thoracic, are confined to the parietes of the thorax, and are named thoracic<unk> .
They pass forward in the<unk>  spaces below the<unk>  vessels. At the back of the chest they lie between the pleura and the posterior<unk>  membranes, but soon pierce the latter and run between the two planes of<unk>  muscles as far as the middle of the rib.
They then enter the substance of the<unk> es interni, and, running amidst their fibers as far as the costal cartilages, they gain the inner surfaces of the muscles and lie between them and the pleura.
Near the sternum, they cross in front of the internal mammary artery and Transversus thoracis muscle, pierce the<unk> es interni, the anterior<unk>  membranes, and Pectoralis major, and supply the integument of the front of the thorax and over the mamma, forming the anterior cutaneous branches of the thorax; the branch from the second<unk> unites with the anterior supraclavicular<unk>  of the cervical plexus.
Numerous slender muscular filaments supply the<unk> es, the Subcostales, the Levatores costarum, the Serratus posterior superior, and the Transversus thoracis. At the front of the thorax some of these branches cross the costal cartilages from one<unk>  space to another.
Lateral cutaneous branches (rami cutanei laterales) are derived from the<unk> , about midway between the vertebræ and sternum; they pierce the<unk> es externi and Serratus anterior, and divide into anterior and posterior branches.
The anterior branches run forward to the side and the forepart of the chest, supplying the skin and the mamma; those of the fifth and sixth<unk>  supply the upper digitations of the Obliquus externus abdominis.
The posterior branches run backward, and supply the skin over the scapula and Latissimus dorsi.
The lateral cutaneous branch of the second<unk> does not divide, like the others, into an anterior and a posterior branch; it is named the intercostobrachial<unk> (Fig. 816). It pierces the<unk> is externus and the Serratus anterior, crosses the axilla to the medial side of the arm, and joins with a filament from the medial brachial cutaneous<unk>  It then pierces the fascia, and supplies the skin of the upper half of the medial and posterior part of the arm, communicating with the posterior brachial cutaneous branch of the radial<unk>  The size of the intercostobrachial<unk> is in inverse proportion to that of the medial brachial cutaneous<unk>  A second intercostobrachial<unk> is frequently given off from the lateral cutaneous branch of the third<unk> ; it supplies filaments to the axilla and medial side of the arm.
The lateral cutaneous branch of the last thoracic<unk> is large, and does not divide into an anterior and a posterior branch.
It perforates the Obliqui internus and externus, descends over the iliac crest in front of the lateral cutaneous branch of the iliohypogastric, and is distributed to the skin of the front part of the gluteal region, some of its filaments extending as low as the greater trochanter.
Intercostal nerves
Phrenic nerve
Vagus nerve
Recurrent laryngeal nerve
Sympathetic trunk
Secondary bronchus
Tertiary bronchus
Bronchiole