<P> 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 mamma, forming the anterior cutaneous branches of the thorax; the branch from the second nerve unites with the anterior supraclavicular nerves of the cervical plexus . </P> <P> The fourth intercostal nerve is enervated by cutaneous slowly - adapting and rapidly - adapting mechanoreceptors, especially by ones densely - packed under the areola; enervation 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 . </P> <P> 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 . </P> <Ul> <Li> Lateral cutaneous branches (rami cutanei laterales) are derived from the intercostal nerves, about midway between the vertebræ and sternum; they pierce the Intercostales externi and Serratus anterior, and divide into anterior and posterior branches . </Li> <Li> 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 . </Li> <Li> The posterior branches run backward, and supply the skin over the scapula and Latissimus dorsi . </Li> </Ul>

Intercostal nerves t1–t5 make up part of which plexus