Initially, ie, during the 4th week and early part of the 5th week of development, the septum transversum lies opposite the 3rd, 4th and 5th cervical somites. During this period tba myoblasts from the myotomes of these somites migrate into the mesenchyme of septum transversum bringing their nerves with them which are derived from the 3rd, 4th, and- 5th cervical segments of the cervical segments of the spinal cord. The nerve fibers from these spinal segments join on each side to form the phrenic nerve. The phrenic nerves pass into the septum transversum through the pleuro-pericardial folds and, thus, these nerves come to tie in 'the fibrous peri.cardium.
Descent of the developing diaphragm occurs due to the rapid growth of the dorsal part of the embryonic body, elongation of the neck, descent of the heart and expansion of the pericardial and pleural cavities. By the 6th week of development the diaphragm reaches the level of thoracic somites. The phrenic nerves now have a descending course. As the diaphragm descends further in a caudal direction, the phrenic nerves are correspondingly lengthened. By the 8th week the dorsal part of the diaphragm lies at the level of the first lumbar vertebra, and by the beginning of the 9th week some of the dorsal muscular bands of the
diaphragm originate from this vertebra. The diaphragm is mainly innervated by the phrenic nerves which supply it with its motor as well as sensory innervation. Since the outer rim of the thoracic wall, the peripheral part of the diaphragmatic musculature arises from the mesenchyme of the thoracic wall, the peripheral part of the diaphragm receives sensory fibers from the lower thoracic (T7 to T12) spinal nerves.