THE THORAX 1031 



external arcuate ligament of the diaphragm. As stated, the right 

 vein opens into the right azygos vein, and the left into the lower 

 left azygos vein. 



Anterior Primary Divisions of the Thoracic Spinal Nerves. — These 

 are twelve in number on each side. The first eleven enter inter- 

 costal spaces, and are called the intercostal nerves. The last, 

 which belongs to the abdomen, lies along the lower border of the 

 twelfth rib, and is called the subcostal nerve (' last dorsal nerve '). 

 At the posterior extremities of the intercostal spaces the intercostal 

 nerves are near the gangliated cord of the sympathetic, and each 

 nerve is connected with the adjacent sympathetic ganglion by two 

 rami comnmnicantes, which are necessarily very short. One of 

 these rami, being composed of spinal fibres, is white, and the other, 

 which consists of sympathetic fibres, is grey. Each intercostal 

 nerve lies below the corresponding artery, and, like it, lies between 

 the parietal pleura and the posterior intercostal aponeurosis, as 

 far as the angle of the rib. Its subsequent course corresponds to 

 that of the artery. The first intercostal nerve is of small size, 

 because the greater part of the anterior primary division of the first 

 thoracic nerve takes part in the formation of the brachial plexus. 

 The second intercostal nerve sometimes gives off a small branch 

 which ascends to join the portion of the anterior primary division 

 of the first thoracic nerve which takes part in the brachial plexus. 

 The further course and distribution of the intercostal nerves belong 

 to the thoracic and abdominal walls, in connection with which they 

 will be found described. 



Thoracic Duct. — ^The thoracic duct commences in the abdomen 

 in a dilatation, called the receptaculum chyli, which is situated in 

 front of the bodies of the first and second lumbar vertebrae, and 

 it terminates by opening into the angle of junction of the internal 

 jugular and subclavian veins of the left side. It is about 18 inches 

 in length, and enters the thorax from the abdomen by passing 

 through the aortic opening of the diaphragm, where it lies between 

 the aorta on the left and the right azygos vein on the right. In 

 this position it ascends in the thorax, resting upon the vertebral 

 column, the right aortic intercostal arteries, and the lower and 

 upper transverse azygos veins, being under cover of the oesophagus. 

 Up to the level of the fourth or fifth thoracic vertebra it occupies 

 the middle line. At this level it passes behind the arch of the 

 aorta, inclining to the left of the middle line. Thereafter it ascends 

 in close contact with the left side of the oesophagus, and behind 

 the thoracic portion of the left subclavian artery. In this position 

 it enters the root of the neck on the left side, where it ascends upon 

 the left side of the oesophagus, between the left common carotid 

 and left subclavian arteries. At about the level of the seventh 

 cervical vertebra it describes a curve, and passes outwards, for- 

 wards, and downwards, in contact with the dome of the left pleura. 

 Thereafter it inclines inwards, and terminates by oj^ening into the 

 angle of junction between the internal jugular and subclavian veins 



