138 THORAX 



between the left pleura and the bodies of the vertebrae, and 

 then between the sympathetic trunk and the vertebral bodies. 



As each artery enters the space to which it belongs it gives 

 off a posterior branch, which passes backwards, between the 

 vertebral column medially and the anterior costo-transverse 

 ligament laterally ; the posterior branch gives off a spinal 

 twig, which enters the vertebral canal through the correspond- 

 ing intervertebral foramen ; then it divides into a medial and 

 a lateral branch which accompany the medial and lateral 

 divisions of the posterior ramus of the corresponding thoracic 

 nerve. After giving off the posterior branch, the trunk of the 

 artery runs laterally, along the upper border of the space to 

 which it belongs, at first anterior to the posterior intercostal 

 membrane, and then between the internal and external 

 intercostal muscles. Its further course has been described 

 already (p. 12). As it passes along the upper border of the 

 intercostal space, in the shelter of the costal groove of the 

 rib, it is situated between the intercostal vein above and the 

 anterior ramus of the thoracic nerve below. 



The Subcostal Arteries. The subcostal arteries are the 

 last pair of branches which spring from the posterior aspect 

 of the thoracic part of the descending aorta. They enter 

 the abdomen by passing behind the lateral lumbo-costal 

 arches, and they run, in company with the last thoracic 

 nerves, along the lower borders of the last pair of ribs in the 

 walls of the abdomen (see p. 408). 



Arterise Intercostales Supremse. The superior intercostal 

 arteries, which supply the upper two intercostal spaces on 

 each side, are derived from the costo-cervical branches of 

 the subclavian arteries (Fig. 6). Each superior intercostal 

 artery commences at the level of the upper border of the 

 neck of the first rib. It descends anterior to the neck of 

 the rib, posterior to the parietal pleura and between the first 

 thoracic ganglion of the sympathetic trunk medially and 

 the first thoracic nerve, which is passing upwards to the 

 brachial plexus, laterally (Fig. 6). At the lower border of 

 the neck of the first rib it gives off the posterior inter- 

 costal artery to the first intercostal space ; then it crosses 

 anterior to the neck of the second rib, and, turning later- 

 ally, it becomes the posterior intercostal artery of the second 

 space. 



Nervi Intercostales. The intercostal nerves are the 



