THE LUMBAR ARTERIES AND VEINS. 375 



the aortic intercostals in their course and distribution. Commonly 

 five in number on each side, they arise from the back of the aorta, 

 and the vessels of opposite sides are sometimes joined in a common 

 trunk. They pass backwards over the hollowed surface of the course; 

 bodies of the last dorsal and upper four lumbar vertebrae, and 

 beneath the pillar of the diaphragm and the psoas, to reach the and termi- 

 interval between the transverse processes, where each ends in an na 

 abdominal and a dorsal branch. The arteries of the right side lie 

 beneath the vena cava. 



The posterior or dorsal branches continue to the back between the a branch to 

 transverse processes, and supply offsets to the muscles and to spinal the back) 

 canal. 



The anterior or abdominal branches are directed outwards, and enter and a 

 the posterior part of the abdominal wall, where they are distributed 



(p. 283). The first lies with the last dorsal nerve across the front of the 

 of the quadratus lumborum, but the others usually pass behind that 

 muscle. Onsets are furnished to the psoas and quadratus muscles, 

 and to the subperitoneal fat, and they anastomose with branches of 

 the renal, capsular, spermatic, right and left colic, and some other 

 visceral arteries. 



The LUMBAR VEINS are the same in number, and have the same The veins 

 course as the arteries. Commencing by the union of a dorsal and an [^arteries 

 abdominal branch at the root of the transverse process, each trunk is and open 

 directed forwards to the vena cava. They open into the back of the jj^ ^ 

 cava, either singly, or conjointly with those of the opposite side. On 

 the left side the veins are longer than on the right, and pass beneath 

 the aorta. 



Around the transverse processes, and beneath the psoas muscles, A plexus 

 the lumbar veins communicate freely with one another, with the transverse 6 

 ilio-lumbar, and with the common iliac, so as to form a plexus of processes. 

 veins. Issuing above from the plexus is a branch, the ascending 

 lumbar vein, which j oins the azygos vein of the corresponding side of 

 the body. 



BEGINNING OF THE AZYGOS VEINS. The azygos vein begins on Origin of 

 each side above the first lumbar vertebra by the above-mentioned V eins^ 

 ascending lumbar vein ; and it is often joined by a branch of com- 

 munication with the inferior cava or the renal vein. The right vein entrance 

 enters the thorax usually with the thoracic duct and the aorta, to the m 

 right of which it lies. The left vein passes through the pillar of the 

 diaphragm, or sometimes through the aortic opening. 



The anatomy of these veins in the thorax is given at p. 483. 



