The Intercostal Space

The intercostal space The intercostal space is the space between two ribs. Since there are 12 ribs on each side, there are 11 intercostal spaces, They are important access points in medicine and surgery. Each intercostal space contains muscles, nerves arteries, veins and lymphatics. Intercostal muscles They are 11 groups on each side; each numbered for the rib above it. In each intercostal space there are 3 groups of intercostal muscles: • External intercostal muscles • Internal intercostal muscles • Innermost intercostal muscles External intercostal muscles They originate on ribs 1-11 and have their insertion on ribs 2-12. They start at the rib tubercle posteriorly and end at the costo-chondral junction anteriorly. They originate the lower border of the rib above and insert into the upper border of the rib below. Anteriorly they become replaced by the anterior intercostal membranes. Their fibres are directed downwards forwards and medially. Responsible for the elevation of the ribs and expanding the transverse dimensions of the thoracic cavity. Internal intercostal muscles They originate on ribs 2-12 and have their insertions on ribs 1-11 They originate from the costal groove of the rib above and insert into the upper border of the rib below. They extend from the margin of the sternum to the angle of the rib where they become replaced by the posterior intercostal membranes. Their fibres are directed downwards backwards Responsible for the depression of the ribs decreasing the transverse dimensions of the thoracic cavity. Innermost intercostal muscles The deep layers of the internal intercostal muscles which are separated from them by the neurovascular bundle. They are composed of: • Transversus thoracis (sternocostalis) muscle • Intercostalis intimus • Subcostalis muscle Innermost intercostal muscles .. Transversus Thoracis Lies anteriorly and formed by 4-5 slips . Each slip arises from the back of the body of the sternum to the back of the costal cartilages of ribs 2-6. Innermost intercostal muscles .. Subcostalis Well developed in the lower part of the thorax. Arises from the inner surface of the rib close to the angle and is inserted to the inner surface of the rib below. This muscle(s) depresses the rib. Intercostal nerves Intercostal nerves 3-7 are typical intercostal nerves. Intercostal nerves 1-2 & 8-12 are atypical. • No. 1: ascends infront of the neck of the 1st rib to join the cervical plexus. • No. 2: its lateral cutaneous branch does not divide. It forms the intercostobrachial nerve. • No. 7-11: are thoracoabdominal; they supply abdominal muscles. • No. 12: is subcostal. Typical intercostal nerves .. 1 They are the nerves in the intercostal spaces 3-6. They are the primary rami of the of the corresponding thoracic nerves. Runs forward between the internal intercostal membrane and the pleura then between the internal and innermost intercostal muscles. Typical intercostal nerves .. 2 At the mid-axillary line, each nerve gives the lateral cutaneous branch. • The lateral cutaneous branch pierces the internal and external intercostal muscles to supply the skin on the side of the thorax. The intercostal nerve continues and at the parasternal line pierces the internal intercostal muscle, the anterior intercostal membrane and the pectoralis major. • It divides into medial and lateral branches to supply the skin on the front of the thorax. Typical intercostal nerves .. 3 (Branches) White ramus communicans to the nearby sympathetic ganglion (from which it receives grey ramus communicans). Collateral muscular branches to the intercostal muscles. Articular branches to the joint of the ribs. Lateral cutaneous branch. Anterior cutaneous branch. Intercostal arteries Anterior intercostal arteries 2 in every space: 1- Upper one anastomoses with the posterior intercostal artery. Posterior intercostal arteries Single, gives collateral branch. 2- Lower one anastomoses with the collateral branch of the posterior intercostal artery. 9 in number: 1- Upper 6 originate from the internal mammary artery. 11 in number and 1 subcostal artery: 1- Upper 2 originate from superior intercostal artery. 2- Lower 3 originate from the musculophrenic artery. 2- From 3-12 originate from the descending aorta. Intercostal veins Anterior intercostal veins: Two in each space Veins of 1-6 spaces end in the internal mammary vein. Veins of 7-9 spaces end in the musculophrenic vein. Posterior intercostal Veins: One in each space 1st ends in the corresponding brachiocephalic vein. 2nd-4th : On the right side: They end in the superior intercostal vein  arch of the azygos vein. On the left side: : They end in the superior intercostal veinleft brachiocephalic vein 5th-12th: On the right side: end separately in the azygos vein. On the left side: 5th-8th: end in the superior hemiazygos vein. 9th-12th: end in the inferior hemiazygos vein. Internal thoracic artery ..1/2 Still called clinically the internal mammary artery. Arises from the lower surface of the first part of the subclavian artery. Descends downwards behind the upper 6 costal cartilages, 1 cm from the margin of the sternum. It divides in the 6th intercostal space into: • Superior epigastric artery. • Musculophrenic artery. Internal thoracic artery ..2/2 Branches Anterior intercostal branches in the upper 6 intercostal space. Perforating branches to the breast (in the 2nd-4th intercostal spaces). Muscular branches to the transversus thoracis. Pericardial branches to the upper part of the pericardium. Mediastinal branches. Pericardio-phrenic branches to accompany the phrenic nerve. Ends at the lower border of the 4th rib by dividing into: musculophrenic and superior epigastric arteries Branches of the musculophrenic artery Anterior intercostal arteries in the intercostal spaces 7-9. Pericaridal branches to the lower part of the pericardium. Muscular branches to the abdominal muscles. Azygos vein Arises in the abdomen either: • From the back of the inferior vena cava at the level of L2. • As a continuation of the right subcostal vein. • By union of the right subcostal and right ascending lumbar veins. Enters the thorax through the aortic opening and ascends upwards. It passes behind the root of the right lung then arches above it to terminate in the superior vena cava at the level of T4. Superior (accessory) hemiazygos vein Receives the posterior intercostal veins in the intercostal spaces 5-8. Descends to the left of the vertebral column till the level of T8. Curves behind the aorta, esophagus and thoracic duct to end in the azygos vein. Inferior hemiazygos vein Arises in the abdomen from either: • Left renal vein. • Union of the left ascending lumbar vein and the left subcostal vein. ascends through the left crus of the diaphragm till the lower border of T8 where it curves to the right to terminate in the azygos vein.