900 HUMAN ANATOMY. 



Relations. — For convenience in description the vena cava inferior may be 

 regarded as consisting of an abdominal and a thoracic portion. The former, which 

 constitutes by far the greater part of its length, has the following relations. Poste- 

 riorly it rests upon the right side of the lumbar vertebrae, upon the origins of the 

 psoas major and minor muscles, and above upon the right crus of the diaphragm ; 

 it crosses in its course the right lumbar and right renal arteries. Medially it is in 

 close relation with the abdominal aorta throughout the greater portion of its course, 

 but separates from it slightly above, the right crus of the diaphragm intervening. 

 Laterally it is in contact with the psoas major muscle below, and at about the middle 

 of its course it is in close relation with the inner border of the right kidney. Ante- , 

 riorly it is covered at its origin by the right common iliac artery and in the lower 

 part of its course by peritoneum. At the level of the third lumbar vertebra it lies 

 beneath the third portion of the duodenum, and immediately above that beneath the 

 head of the pancreas and the main stem of the portal vein, which crosses it obliquely. 

 Finally, it lies in the vena caval fissure of the liver, having to the right the right 

 lobe and to the left the Spigelian lobe, and being sometimes completely surrounded by 

 liver^issue, owing to a thin portion of it bridging over the fissure. Throughout this 

 part of its course it is firmly united to the walls of the fissure by fibrous bands. 



In its thoracic portion, which is quite short, measuring not more than 3 cm. in 

 length, it is in relation at first with the right lung and pleura, and in the upper part 

 is enclosed for about 1.2 cm. in the pericardium. 



Variations. — The development of the inferior vena cava (page 927) shows it to be formed 

 by the union of three primarily distinct structures. Its upper part, between the entrance of the 

 hepatic veins and the right auricle, is the upper part of the embryonic ductus venosus, then fol- 

 lows a considerable portion derived from the right subcardinal vein, and, finally, its lower part is 

 formed from the right cardinal vein. Of these embryonic veins the ductus venosus is unpaired, 

 the other two are the right members of paired veins, whose fellows undergo almost complete 

 degeneration. 



Anomalies of the vena cava, which are not uncommon, are for the most part e.xplicable as 

 a persistence or modification of the embryonic conditions. Thus, that portion of the vessel which 

 is formed from the right subcardinal and right cardinal may fail to develop, in which case what 

 is termed a persistence of the cardinals occurs. Up to a point above the level of the renal veins 

 the vena cava is represented by two parallel trunks lying one on either side of the aorta, the one 

 receiving the right common iliac vein and the other the left. These represent the abdominal 

 portions of the cardinal veins or, in the majority of cases, more probably the subcardinals, and unite 

 above with the unpaired ductus venosus, which carries their blood to the heart. In other words, 

 such cases are, as a rule, to be regarded as a similar development of both subcardinal veins. 



Occasionally, however, the development of the right subcardinal to form the vena cava 

 may proceed as usual, but it fails to make a connection with the ductus venosus, one of its con- 

 nections with the right cardinal enlarging so that this vein receives the caval blood, carries it 

 through the aortic opening of the diaphragm, and, as the azygos vein, empties it into the superior 

 vena cava. The hepatic veins open as usual into the ductus venosus, which passes to the right 

 auricle in the normal manner, and the vena cava inferior is thus represented by two distinct veins, 

 the upper part of the ductus venosus. whicfi in such cases is termed the common hepatic vein {v. 

 hepatica coinniunis), and the subcardinal and cardinal portion. 



Another variation may be produced by a reversal of the roles of the two subcardinals in 

 forming the vena cava, the left being the one which develops, while the right degenerates. 

 Such a condition is found in all cases of situs inversus viscerum, but it has also been observed 

 in cases in which there was otherwise a normal arrangement of the organs. In such cases the 

 vena cava in the lower part of its course lies to the left of the aorta instead of to the right, and at 

 the level of the renal arteries it crosses to the right side in front of the aorta, its further course 

 being normal. But just as the lower part of the inferior vena cava, when normally formed from 

 the right subcardinal, may fail to unite with the ductus venosus but retain its primary connection 

 with the azygos, so, too, when formed from the left subcardinal, it may retain its connection with 

 the hemiazygos and drain through that vessel into the azygos and so into the superior vena cava. 



These various cases include the principal variations which occur in connection with the 

 vena cava inferioi. It may be pointed out that normally connections exist between the azygos 

 vein and the vena cava below the diaphragm ; by means of the ascending lumbar veins, and 

 also by the thoraco-epigastric veins, connection is established between tributaries of the inferior 

 cava and the external iliac veins, and the axillary vein. By means of these normally subordinate 

 channels opportunity is afforded for the maintenance of the circulation in case of obliteration of 

 the vena cava. 



Practical Considerations. — The inferior cava may be ruptured in severe 

 abdominal injuries, as in the case of a weight falling upon, or a wagon passing over, 

 the belly. The site of rupture is most often in the portion lying in the hepatic 



