338 DISSECTION OF THE THORAX. 



to the ribs. About the centre (from front to back) of the intercostal space 

 a superficial twig arises, which accompanies the cutaneous nerve. 



The highest artery of the aortic set of intercostals anastomoses with the 

 superior intercostal branch of the subclavian artery; and the lowest (below 

 the true ribs) enters the abdominal wall, and anastomoses with the arteries 

 of that part. 



The posterior branch turns backwards between a vertebra and an as- 

 cending costo-transverse ligament, and is distributed in the Back. As it 

 passes by the intervertebral foramen it furnishes a small spinal branch to 

 the vertebra? and the spinal cord. (See VESSELS OF THE SPINAL CORD.) 



The intercostal vein resembles closely the artery in its course and 

 branching. Near the head of the rib it receives a contributing dorsal 

 branch, and then joins an azygos vein. 



The superior intercostal artery of the subclavian trunk (p. 78) descends 

 over the neck of the first rib, external to the ganglion of the sympathetic, 

 and supplies a branch to the first intercostal space : continuing to the 

 second space, which it supplies in like manner, it anastomoses with the 

 upper aortic branch. 



Its intercostal offsets divide into an anterior and a posterior branch, like 

 the arteries from the aorta. 



The vein accompanying the artery opens into the innominate vein of 

 the same side. The left superior intercostal vein (fig. 105, *), formed by 

 branches from the two or three highest spaces, is joined by the left bron- 

 chial vein, and ends in the left innominate vein, after crossing the arch of 

 the aorta. 



The INTERCOSTAL or AZYGOS VEINS are two in number on the left, and 

 one on the right side, and receive branches corresponding with the offsets 

 furnished by the thoracic aorta. 



The right or larger azygos (fig. 106, 3 ) begins in the lumbar veins on 

 the right side of the spine, and its origin is discribed witli the vessels of 

 the abdomen. It enters the thorax through the aortic opening of the 

 diaphragm, and ascends on the right side of the thoracic duct, over the 

 intercostal arteries and the bodies of the vertebra?. Opposite the fourth 

 intercostal space the vein arches forwards above the root of the right lung, 

 and enters the superior cava as this vessel pierces the pericardium. Its 

 valves are very incomplete, so that blood may flow either way ; and the 

 intraspinal and intercostal veins may be injected through it. 



Branches. In this vein are collected the intercostals of the right side 

 below the upper two spa,ces ; some of the intercostals of the left side of 

 the thorax, through the left azygos veins ; and some small O3sophageal, me- 

 diastinal, and vertebral veins, with the right bronchial vein. 



By means of the right vein the inferior communicates with the superior 

 cava, so that the blood may reach the heart from the lower part of the 

 body, or the opposite, if one of the cava3 should be obstructed. 



The left lower azygos vein (fig. 100, *) begins in the abdomen in the 

 lumbar veins of the left side of the vertebral column. Entering the thorax 

 along with the aorta, or through the crus of the diaphragm, the vein as- 

 cends on the left of the aorta as high as the seventh or eighth dorsal ver- 

 tebra, where it crosses beneath that vessel and the thoracic duct to end in 

 the right azygos. It receives the four or five lower intercostal veins of the 

 left. >ide, and some O3sophageal and mediastinal branches. 



The left upper azygos vein (fig. 100, 5 ) is formed by offsets from the 

 space between the superior intercostal and the preceding. Receiving three 



