The anterior divisions of the second, third, fourth, fifth, and sixth thoracic nerves, and the small branch from the first thoracic, are confined to the walls of the thorax, and are named thoracic intercostal nerves. They pass forward in the intercostal spaces below the intercostal vessels. At the back of the chest they lie between the pleura and the posterior intercostal membranes, but soon they run between the internal intercostals and the innermost intercostals then anteriorly they lie between the pleura and the internal intercostals. Near the sternum, they cross in front of the
internal mammary artery and
transversus thoracis muscle, pierce the
intercostales interni, the anterior intercostal membranes, and
pectoralis major, and supply the integument of the front of the thorax and over the mammary gland, forming the anterior cutaneous branches of the
thorax; the branch from the second nerve unites with the anterior
supraclavicular nerves of the
cervical plexus. The
fourth intercostal nerve is innervated by cutaneous slowly-adapting and rapidly-adapting
mechanoreceptors, especially by ones densely-packed under the areola; innervation subsequently triggers
oxytocin release, which, when in the peripheral bloodstream, causes
myoepithelial cell contraction and
lactation: this is an example of a non-nerve-innervation muscular reflex.
Branches Numerous slender muscular filaments supply the
Intercostales, 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 intercostal space to another. •
Lateral cutaneous branches (rami cutanei laterales) are derived from the intercostal nerves, about midway between the vertebrae and sternum; they pierce the
Intercostales 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 and skin, fourth nerve anterior branches supplying the
areola and the mamma; those of the fifth and sixth nerves 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 intercostal nerve does not divide, like the others, into an anterior and a posterior branch; it is named the
intercostobrachial nerve. ==The lower thoracic nerves: 7th–11th==