396 COMPARATIVE ANATOMY 



In the thoracic region are the azygos, hemiazygos, accessory hemi- 

 azygos, ascending lumbar, vertebral, and internal mammary veins. The 

 azygos vein extends along the thoracic vertebral column, receiving blood 

 from the right intercostal veins, from the hemiazygos vein and from the 

 accessory hemiazygos vein. It opens anteriorly into the precava. The 

 hemiazygos vein receives blood from the left intercostal veins, and joins the 

 azygos at about the level of the eighth rib. The accessory hemiazygos vein 

 crosses the backbone from left to right just above the hemiazygos 

 vein. In the region below the diaphragm, the azygos and hemiazygos 

 veins continue as right and left ascending lumbar veins. The vertebral 

 column in the thoracic and lumbar regions is drained by the vertebral, 

 intercostal, lumbar, sacral, and intervertebral veins. The internal 

 mammary veins drain the superficial veins of the thorax as well as the 

 anterior intercostal, bronchial, and pericardiac veins and open into the 

 right and left innominate veins. 



The chief superficial veins of the upper extremity are the cephalic 

 and basilic veins. The cephalic vein joins the axillary vein below the 

 clavicle. The basilic vein unites with the brachial vein at about the 

 middle of the upper arm. The deeper veins of the arm, forearm, and 

 hand parallel the arteries of these regions. Each artery, however, is 

 accompanied by a pair of veins, venae comitantes. Beginning with the 

 volar veins of the hands, the veins of the upper extremity, in order, are the 

 metacarpal, ulnar and radial, brachial, axillary, and subclavian. The trib- 

 utaries of these veins correspond closely in number and relations with 

 arteries of the same name. 



Most of the blood from the legs, abdomen, and viscera is returned 

 to the right auricle by the postcava. The tributaries of the postcava 

 are the right and left common iliac veins, which return blood from the legs, 

 the lumbar veins from the lumbar body-wall, the right spermatic or 

 ovarian vein from the gonad, the renal veins from the kidneys, the right 

 adrenal vein (the adrenal vein of the left side and the left spermatic or 

 ovarian veins are tributaries of the left renal vein), the inferior phrenic 

 veins, of which the left usually opens into the left renal vein, and the 

 hepatic veins which are largest of all. 



The portal vein is unique in having capillaries at both of its ends. 

 The portal collects blood from the intestine, stomach, and spleen and 

 deUvers it into the capillaries (sinusoids) of the liver. Its branches 

 parallel those of the mesenteric arteries and some of those of the celiac 

 artery. The chief tributaries of the portal vein are the superior mesenteric 

 vein, the splenic vein which drains the spleen, stomach, and pancreas, 

 the inferior mesenteric vein (the relations of which correspond to those 

 of the artery of the same name), the coronary or gastric vein from the 



