AZYGOS AXD SriXAL VEINS. 471 



p. 455) is mcst frequently united with the left innominate vein, bnt some- 

 times is connected with the main azygos vein. 



Of the remaining left intercostal veins, one or two, generally about the 

 fifth and sixth, pass directly into the azygos ; while the lowest in greater 

 number unite almost constantly into one trunk, forming the left or small 

 azygos, which crosses to join the main azygos in the neighbourhood of the 

 seventh dorsal vertebra. There is frequently union between these three 

 sets of veins or their intercostal branches, so that a part of one may be 

 replaced by another, and the relative size of the veins may be subject to 

 considerable variation. 



The left lower or small azygos vein (vena hemiazygos) commences from 

 on a of the lumbar veins (ascending lumbar), or from the left renal vein, and, 

 having entered the thorax with the aorta, or through the crus of the 

 diaphragm, ascends upon the spins in front of the left intercostal arteries, 

 receiving the lower intercostal veins of the left side ; and passing behind the 

 aorta, it opens into the right azygos vein, opposite the sixth or seventh 

 dorsal vertebra. 



The azygos vein has been seen to receive the lower vena cava, and, in such cases, is 

 of course extremely large. 



In one instance, Meckel found the azygos ending in the subclavian vein. 



All the intercostal veins of the left side have been observed in some instances to 

 join a single vein, which ended in the left innominate ; the arrangement corresponding 

 with that on the right side of the body. 



The bronchial veins return the blood employed in the nutrition of the 

 lungs. Their course corresponds with that of the bronchi, which suppott 

 them as they pass towards the root of the lungs. The bronchial vein of the 

 right side opens into the trunk of the azygos vein near its termination, that 

 of the opposite side ends in the superior intercostal vein. 



VEINS OF THE SPINE. 



The spinal veins form plexuses of closely anastomosing vessels along the 

 whole length of the spinal column. They have no valves. 



The veins within and upon the spinal column may be distinguished into 

 the following sets : a. The dorsal, placed deeply in the veitebral grooves, 

 and resting upon the spines and arches of the vertebrae : 6. The veins 

 lodged within the bodies of the vertebrae : c. The anterior longitudinal, 

 two long series of veins, or rather venous plexuses, extend behind the 

 bodies of the vertebrae in the whole length of the canal : d. The posterior 

 longitudinal veins, situated within the canal on the fore part of the 

 arches of the vertebras : e. The veins of the spinal cord. There are like- 

 wise branches of communication, some of which connect all the other sets 

 together, and some which bring them into connection with the general 

 venous system. (Breschet, " Essai sur les Veines du Rachis," 4to. ; 

 " Traite' Anatomique sur le Systeme Veineux," fol., 1829 ; Cloquet, 

 " Traite d'Anatomie descriptive," <fec.) 



a. The dorsal veins. The Wood from the muscles and integument alcrig 

 the back of the spine is returned by a series of short veins, which ramify 

 upon the arches and spinous processes of the vertebrae. They run forwards 

 close to the spinous proctsses, and on reaching the interval between the 

 arches of the vertebrae, pierce the ligamenta subflava, and terminate in a 

 venous plexus within the canal. Towards the outer part of the interverte- 

 bral grooves other veins arise, which pass obliquely forwards, through the 

 in tertrans verse spaces, in company with the posterior branches of the lumbar 



