ABNOEMALITIES OF VEINS. 1059 



The posterior facial vein may terminate entirely in the common facial vein, or in the 

 external or the internal jugular vein. It may be very small, and occasionally it is absent. 



Variations of the cranial blood sinuses are not numerous. One transverse sinus may be 

 absent or very small, when, as a rule, that of the opposite side is enlarged. The inferior sagittal, 

 the occipital, or the spheno-parietal sinuses may be absent, and there may be an additional petro- 

 squamous tributary to the transverse sinus. The petro-squamous sinus, when present, is the 

 remains of a sinus which crossed the temporal bone, passed through the post-condyloid foramen 

 and terminated in the lateral cerebral vein. In the human adult, in rare cases, it pierces 

 the skull behind the condyle of the mandible, and terminates in the posterior facial vein. This 

 is the normal arrangement in some mammals. 



THE VEINS OF THE SUPERIOR EXTREMITY. 



The superficial veins of the forearm are extremely variable ; any of them may be absent, 

 but most commonly it is the median or the cephalic vein which is wanting. The median cephalic 

 and the cephalic veins may be small or absent, and, on the other hand, the cephalic vein may be 

 larger than usual. Moreover the cephalic vein may end in the external jugular vein, its original 

 termination ; or it may be connected with the external jugular vein by an anastomosing channel 

 which sometimes passes over the clavicle and sometimes through that bone. 



The basilic vein is sometimes larger and sometimes smaller than usual, and it may pierce the 

 fascia of the arm at a more proximal or at a more distal level than usual. 



The venae comites of the arteries of the upper extremity generally terminate at the lower 

 border of the subscapularis, where they join the axillary vein, but they may end above or below 

 the position of their usual termination. 



The subclavian vein sometimes passes behind instead of in front of the scalenus anterior 

 .e, and it has been seen passing between the clavicle and the subclavius muscle. 



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THE INFERIOR VENA CAVA. 



The lower part of the inferior vena cava is sometimes absent, in which case the common iliac 

 veins ascend, one on the right and the other on the left of the aorta, to the level of the second 

 lumbar vertebra, where the left common iliac vein receives the -left renal vein, and then crosses in 

 front of or behind the aorta to fuse with the corresponding vein of the right side ; in such cases, 

 therefore, the inferior vena cava commences at the level of the second lumbar vertebra, and it 

 represents only the upper and last-formed part of the ordinary vessel ; the common iliac veins, 

 each of which receives the lumbar veins of its own side, are exceptionally long, and they may 

 or may not be united at the pelvic brim by a small transverse anastomosing channel. Cases of 

 this kind are sometimes described as partial doubling of the inferior vena cava. 



Occasionally the inferior vena cava does not terminate in the right atrium, but is continuous 

 with the vena azygos, which is much enlarged, all the inferior caval blood being then carried 

 to the superior vena cava. In such cases the hepatic veins open directly into the right atrium 

 without communicating with the inferior vena cava. 



The lower part of the inferior vena cava sometimes lies to the left instead of to the right of 

 the aorta ; this condition is associated with a long right common iliac vein, which crosses obliquely 

 from right to left to join the shorter left common iliac vein. After receiving the left renal vein 

 the misplaced inferior vena cava crosses in front of the aorta, reaching the right side at the level of 

 the second or first lumbar vertebra. In other cases, however, the left inferior vena cava continues 

 upwards through the left cms of the diaphragm, usurping the place of a greater or smaller part 

 of the hemiazygos vein ; having entered the thorax, it may cross to the opposite side and terminate 

 in the vena azygos, or it may continue upwards on the same side, and after arching over the 

 root of the left lung, descend behind the left atrium to terminate in the right atrium in the 

 situation of the coronary sinus. In this group of cases also the hepatic veins open separately into 

 the right atrium. 



The inferior vena cava may lie ventral instead of dorsal to the right internal spermatic artery, 

 in which case the lower part of the vessel has been derived from the subcardinal vein instead of 

 from the posterior cardinal vein. (Johnston, Journ. of Anat. and Phys. xlvii. 1913.) 



The tributaries of the inferior vena cava are also subject to variation. Additional renal, 



spermatic, ovarian, or suprarenal veins may be present. Two or three lumbar veins of one or 



oth sides may unite into a common trunk which terminates in the inferior vena cava, and the 



hepatic veins may open separately, or after fusing into a common trunk, into the right atrium 



near the opening of the inferior vena cava. 



No explanation of the variations of the inferior vena cava and its tributaries is necessary, 



beyond the statement that they are due to persistence of portions of the cardinal and subcardinal 



ems which usually disappear, and to the persistence of transverse anastomoses and tributaries 



which usually atrophy, or to modifications of those which ordinarily take part in the formation 



the inferior vena caval system. 



The left common iliac vein is short and the right long when the inferior vena cava lies on 

 e left side. The common iliac veins may be absent, the hypogastric veins uniting to form the 

 commencement of the inferior vena cava, into which the external iliac veins open as lateral 

 tributaries. 



