THE VENA CAVA INFERIOR 693 



enters the right atrium by way of the coronary sinus, collecting the coronary veins. Cases are 

 on record of a left superior cava terminating in the left atrium. 



The azygos veins. — Variations of these veins and of the intercostal veins have been dealt 

 with on pp. 663-664. For their morphogenesis, see under vena cava inferior. 



The veins of the neck, face, and scalp. — These veins have so many variations in detail that 

 it is difficult, in the case of some veins, to assign their normal distribution. The external 

 jugular, for instance, is usually described in Enghsh text-books as a tributary of the subclavian 

 vein; it is assigned by the BNA to the internal jugular. It is frequently found to open into the 

 angle between the two, or, forming a plexus with its tributaries, drain into both. The origin 

 of the external jugular vein is also exceedingly variable. The external jugular may be small, 

 or absent, in which case the anterior jugular is large. The reverse may be the case since the 

 external jugular frequently receives the posterior, and sometimes the common facial. Fortu- 

 nately venous variations are not of prime surgical importance. 



Veins of the cranium. — The venous sinuses of the dura mater are not subject to important 

 variations. Variations in the relative size of the transverse stiluses have been referred to on 

 p. 651. The petrosquamous sinus, occasionally present, is described on p. 653. The occipital 

 and inferior sagittal sinuses are frequently absent. 



The cerebral veins are hable to great variation in detail: the great cerebral vein may be absent, 

 as a single trunk, in which case the internal cerebral veins open directly into the sinus rectus. 

 The middle cerebral vein may open into the sphenoparietal, or superior petrosal sinus or into the 

 basilar plexus. 



Veins of the upper extremity. — The subclavian vein is occasionally posterior to the artery, 

 or spUts to enclose the latter and the anterior scalenus. Either case represents a partial re- 

 tention of the early condition in which the vein passes behind the brachial plexus. Variations 

 in the superficial veins have been referred to on p. 668. The question of the most common 

 distribution of these vessels has lately been fully reviewed by Ben-y and Newton. The cephalic 

 vein occasionally opens into the external jugular by persistence of the embryonic jugulo-cephalic 

 vein. 



B. THE VENA CAVA INFERIOR AND ITS TRIBUTARIES 



1. MORPHOGENESIS 



The right and left post-cardinal veins (fig. 544) are at first symetrical in size and position. 

 Early in development each posterior cardinal vein becomes involved in the growth of the cor- 

 responding mesonephros, and the original venous channel is converted into a system of sinusoids. 

 In the sinusoidal circulation of each mesonephros two main longitudinal venous channels soon 

 make their appearance. One lies ventro-medial to the mesonephros and is called the sub-cardinal 

 vein. The other, which lies dorsal to the mesonephros, receives the segmental veins and is 

 frequently called the post-cardinal. Since the mesonephric segment of the post-cardinal vein 

 has obviously passed out of existence, the vein in question (unlabelled in fig. 544) will be here 

 distinguished as the dorsal trunk. The sub-cardinals communicate freely between themselves 

 and with the dorsal trunks, Ue ventral to the mesonephric arteries, and are at first symmetrical. 

 The cephalic end of the right sub-cardinal now acquires a communication with the common 

 hepatic vein, thus providing a new means of drainage for the sub- and post-cardinal systems 

 (fig. 544) . The rapidly enlarging main venous channel resulting from this alternative method of 

 drainage follows the right dorsal trunk as far as the level of the permanent renal veins. It is 

 then transferred, by means to an anastomosing channel, to the right sub-cardinal and, through 

 this, to the common hepatic vein; it becomes the vena cava inferior. From now on the portions 

 of the sub-cardinal veins not participating in the formation of the cava dwindle rapidly. A 

 cross anastomosis between the right and left sub-cardinals persists as the portion of the adult 

 left renal vein which crosses ventral to the aorta. By means of it the remainder of the left 

 renal; the left internal spermatic and left suprarenal veins are connected with the vena cava. 

 The left lumbar and left common iliac veins are also transferred to the vena cava, probably by 

 direct anastomosis with the left post-cardinal vein. The vena cava inferior is at first lateral 

 to the right ureter, its transference to the medial side occurs through anastomosis. 



The portion of the right posterior cardinal vein above the mesonephric region, together with 

 its continuation into the dorsal trunk, becomes the azygos vein (fig. 544). The corresponding 

 vessel upon the left side is transformed into the accessory hemiazygos and hemiazygos veins. 

 The hemiazygos vein is drained into the azygos by means of an anastomosing channel which 

 may also drain the accessory hemiazygos. The variability of the means of drainage of the 

 accessory hemiazygos vein, by means of anastomosing channels, is referred to on p. 663. The 

 ascending lumbar veins are anastomosing channels of new formation. 



In the lower extremity, as in the upper, the original superficial plexus is gradually drained 

 by a loop-hke marginal vein. The fibular limb of this loop, the primitive fibular vein, becomes 

 small saphenous; it follows the sciatic nerve and opens into the post-cardinal. The next vein to 

 be developed is the great saphenous; the small saphenous is transferred to this by an anastomos- 

 ing vein which is usually present in the adult — the femoro popliteal vein. The deep veins are 

 of later formation. The drainage of the small saphenous is usually taken over bj' the popliteal 

 vein. 



2. VARIATIONS 



In determining the probable embryonic cause of variations of the vena cava inferior the 

 possibihty of abnormal persistence of the sub-cardinal veins must be remembered. The posi- 

 tion of transverse anastomoses with regard to the aorta is often the key to diagnosis. Instruc- 



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