THE THORACIC AORTA 



589 



the accessory hemiazygos vein when this is present. They are all covered by 

 the left pleura and lung (fig. 478). 



The branches of the intercostal arteries are: — (a) anterior, (b) posterior. 



(a) The anterior branches [rami anteriores] at first cross the intercostal space obliquely, in 

 consequence of the downward direction of the ribs, toward the angle of the rib above, and 

 thence are continued forward in the costal groove, and anastomose with the superior branches 

 of the anterior intercostals from the internal mammary in the upper spaces, and from the 

 musculo-phrenic in the lower spaces. They he at first on the external intercostal muscles, 

 being covered in front by the pleura and lung, the endothoracic fascia, and the subcostal 

 muscles. Opposite the heads of the ribs they are crossed by the sympathetic nerve. At the 

 angle of the ribs they pass under cover of the internal intercostal muscles, and thence to their 

 termination he between the two intercostal muscles. Their situation in the midspace as far 

 as the angle of the rib should be remembered in performing paracentesis thoracis. To avoid 

 the risk of injuring the vessels, the puncture should not be made further back than the angle 

 of the ribs. They are accompanied by a nerve and vein, the vein lying above and the nerve 

 below, except in the upper spaces, where the artery, having to ascend to reach the space, at 

 first lies below the nerve which runs more horizontally. The uppermost branch anastomoses 

 with the costo-cervical artery from the subclavian, and at times supplies almost entirely the 

 second intercostal space. The arteries to the tenth and eleventh spaces on reaching the end 



Fig. 478. — Scheme of Intercostal Artery. 



Longissimus dorsi 



(Walsham.) 



Medial cutaneous branch 

 Semispinalis dorsi and multifidus spinas 



Posterior spinal 

 arteries 



Prelaminar branch 



Neural branch 



Postcentral branch 



Spinal cord 



Anterior spinal artery 



Lateral cutaneous branch 

 Ilio-costalis 



Spinal branch 



Posterior branch 



Intercostal artery 



Vena hemiazygos 



Vena azygos 

 Thoracic duct 



(Esophagus 



Anterior intercostal 



Internal mammary artery- 

 Anterior cutaneous branch 



Sympathetic 



Collateral branch 



Anterior branch 

 of aortic intercostal 



Lateral cutaneous 

 branch 



Lower branch of an- 

 terior intercostal 



Medial mammary 



branch 

 Upper or main branch 

 of anterior intercostal 



of their respective ribs pass between the abdominal muscles, and anastomose with the inf. 

 epigastric artery from the external ihac, and with the lumbar arteries from the abdominal 

 aorta. The artery beneath the tweKth rib anastomoses with the lumbar arteries and with 

 the external chcumflex ihac. 



Each anterior branch gives off the following: — (i) The collateral branch which comes off 

 near the angle of the rib and runs forward, between the external and internal intercostals, 

 along the upper border of the lower rib enclosing the space. It is smaller than the main anterior 

 branch and anastomoses with the lower anterior intercostal in each space, (ii) Muscular 

 branches [rami musculares] supply the intercostal, pectoral and abdominal muscles, (iii) 

 The lateral cutaneous branches [rami cutanei laterales], both pectoral and abdominal, run with 

 the corresponding branches of the intercostal nerves through the external intercostal and ser- 

 ratus anterior muscles. They then divide into anterior and posterior branches which turn for- 

 ward and backward, respectively, to supply the integument. The anterior branches from the 

 third, fourth and fifth spaces supply lateral rnammarij branches [rr. mammarii laterales] to the 

 lateral region of the breast, (iv) Anterior cutaneous branches [rami cutanei anteriores] 

 pierce the external intercostal hgament and the pectorahs major just lateral to the sternum. 



