THE DEVELOPMENT OF THE VASCULAR SYSTEM 289 



atrium does not proceed far enough to cause all four of the pulmonary veins 

 to open separately (see p. 236). The upper (more cephalic) vein on the right 

 side may open into the superior vena cava ; or the upper vein on the left side 

 may open into the left innominate vein. A possible explanation for this is 

 that the pulmonary veins are formed after the heart and other vessels have 

 developed to a considerable degree, and some of them may unite with the 

 other vessels instead of with the atrium. 



Occasionally two superior vena cava are met with. In this case the right 

 opens into the right atrium in the normal position; the left opens into the 

 right atrium through the coronary sinus which naturally is much enlarged. 

 This condition represents a persistence of the proximal end of the left 

 anterior cardinal vein and the left duct of Cuvier, and is the normal arrange- 

 ment in many of the lower Vertebrates. Even with two venae cavae there 

 may be a small anastomosing branch in the position of the left innominate 

 vein, which represents the normal structure in the Marsupials (see Figs. 

 232 and 233 and p. 254). There are a few cases on record of a single left 

 superior vena cava. 



The inferior vena cava is also subject to variations which represent the 

 abnormal persistence of certain embryonic vessels. Perhaps the most 

 striking of these variations is the condition known as double inferior vena 

 cava. There may be two parallel vessels, of equal or unequal size, which 

 unite at or above the level of the renal veins. This condition is to be ex- 

 plained by the persistence of parts of both posterior cardinal veins. It is 

 met with not infrequently among the lower Mammals, especially the Mar- 

 supials (see Figs. 233 and 236). 



Rarely the inferior vena cava opens into the superior, and in this case the 

 hepatic veins open directly into the right atrium. This anomaly probably 

 represents a failure of the absorption of the sinus venosus into the wall of the 

 atrium (p. 236). 



A left renal vein may open into the left common iliac, which condition 

 represents a persistence of the more caudal part of the left posterior cardinal 

 (Fig. 236). This anomaly is rare. 



The azygos vein occasionally presents variations which are due to anoma- 

 lous development. All the intercostal veins on the left side may be collected 

 into a vessel which opens into the left innominate vein. There may be a 

 single median azygos vein; or there may be a transposition of the azygos vein. 

 It may be on the left side and open into the coronary sinus (normal condi- 

 tions in the sheep and a few other Mammals). The latter condition repre- 

 sents a persistence of the more cephalic part of the left posterior cardinal 

 vein (see Figs. 233 and 234). 



