288 TEXT-BOOK OF EMBRYOLOGY. 



be right common carotid, left common carotid, left subclavian, right subclavian. 

 In this case the proximal part of the right subclavian represents the portion of 

 the right dorsal aortic root just cranial to the bifurcation; the fourth arch on 

 the right side disappears, (c) Or very rarely the order may be right subclavian, 

 right common carotid, left common carotid, left subclavian. 



5. Five branches of the arch are rare. In order they are right subclavian, 

 right vertebral, right common carotid, left common carotid, left subclavian. 



6. Very rarely there are six branches of the arch; right subclavian, right 

 vertebral, right common carotid, left common carotid, left vertebral, left 

 subclavian. 



Anomalies of the Veins. — The two pulmonary veins on each side, more 

 frequently those on the left side, may unite into a common trunk before opening 

 into the atrium. This variation is probably due to the fact that the absorption 

 of the originally single pulmonary trunk into the wall of the atrium does not 

 proceed far enough to cause all four of the pulmonary veins to open separately 

 (see p. 231). 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 car- 

 dinal vein and the left duct of Cuvier, and is the normal arrangement in many 

 of the lower Vertebrates. Even with two venae cava? there may be a small anas- 

 tomosing branch in the position of the left innominate vein, which represents 

 the normal structure in the Marsupials (see Figs. 232 and 233 and p. 256). 

 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 explained by the persistence 

 of parts of both posterior cardinal veins. It is met with not infrequently among 

 the lower Mammals, especially the Marsupials (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. 231). 



