The intercostal nerves end as anterior cutaneous branches, which emerge either parasternally, between adjacent costal cartilages, or laterally to the midline, on the anterior abdominal wall, to supply the skin.
In addition to these major branches, small collateral branches can be found in the intercostal space running along the superior border of the lower rib.
In the thorax, the intercostal nerves carry: somatic motor innervation to the muscles of the thoracic wall (intercostal, subcostal, and transversus thoracis muscles), somatic sensory innervation from the skin and parietal pleura, and postganglionic sympathetic fibers to the periphery.
Sensory innervation of the skin overlying the upper thoracic wall is supplied by cutaneous branches (supraclavicular nerves), which descend from the cervical plexus in the neck.
In addition to innervating the thoracic wall, intercostal nerves innervate other regions:
The anterior ramus of T1 contributes to the brachial plexus.
The lateral cutaneous branch of the second intercostal nerve (the intercostobrachial nerve) contributes to cutaneous innervation of the medial surface of the upper arm.
The lower intercostal nerves supply the muscles, skin, and peritoneum of the abdominal wall.
The diaphragm is a thin musculotendinous structure that fills the inferior thoracic aperture and separates the thoracic cavity from the abdominal cavity (Fig. 3.34 and see Chapter 4). It is attached peripherally to the: xiphoid process of the sternum, costal margin of the thoracic wall, ends of ribs XI and XII, ligaments that span across structures of the posterior abdominal wall, and vertebrae of the lumbar region.
From these peripheral attachments, muscle fibers converge to join the central tendon. The pericardium is attached to the middle part of the central tendon.
In the median sagittal plane, the diaphragm slopes inferiorly from its anterior attachment to the xiphoid, approximately at vertebral level TVIII/IX, to its posterior attachment to the median arcuate ligament, crossing anteriorly to the aorta at approximately vertebral level TXII.
Structures traveling between the thorax and abdomen pass through the diaphragm or between the diaphragm and its peripheral attachments:
The inferior vena cava passes through the central tendon at approximately vertebral level TVIII.
The esophagus passes through the muscular part of the diaphragm, just to the left of midline, approximately at vertebral level TX.
The vagus nerves pass through the diaphragm with the esophagus.
The aorta passes behind the posterior attachment of the diaphragm at vertebral level TXII.
The thoracic duct passes behind the diaphragm with the aorta.
The azygos and hemiazygos veins may also pass through the aortic hiatus or through the crura of the diaphragm.
Other structures outside the posterior attachments of the diaphragm lateral to the aortic hiatus include the sympathetic trunks. The greater, lesser, and least splanchnic nerves penetrate the crura.
The arterial supply to the diaphragm is from vessels that arise superiorly and inferiorly to it (see Fig. 3.34). From above, pericardiacophrenic and musculophrenic arteries supply the diaphragm. These vessels are branches of the internal thoracic arteries. Superior phrenic arteries, which arise directly from lower parts of the thoracic aorta, and small branches from intercostal arteries contribute to the supply. The largest arteries supplying the diaphragm arise from below it. These arteries are the inferior phrenic arteries, which branch directly from the abdominal aorta.