THE HUMAN EMBEYO. 



and does not appear until the hind-limbs have begun to become 

 prominent. It joins the ductus venosus as this emerges from 

 the liver. 



The Cuvierian veins. Each Cuvierian vein (Fig. 198, vo) is 

 formed by the junction of an anterior and a posterior cardinal 

 vein. The anterior cardinal vein persists as the external jugular 

 vein, and is joined later on by the internal jugular and sub- 

 clavian veins. 



The posterior cardinal veins disappear, in the middle part 

 of their course, on the replacement of the Wolffian bodies, with 

 which they are specially related, by the permanent kidneys. 

 The hinder ends of the veins become the internal iliac veins, and 

 acquire connections with the allantoic veins. The anterior 

 portion of the right posterior cardinal vein gives rise to the 

 azygos vein. 



The Cuvierian veins themselves run at first transversely ; but, 

 as the heart shifts backwards, their direction becomes at first 

 oblique, and finally longitudinal. 



The right Cuvierian vein persists as the anterior vena cava. 

 The left Cuvierian vein undergoes important changes : up to 

 the end of the second month it is as large as the right vein ; 

 but during the third month a communicating vessel is formed 

 between the left and right Cuvierian veins, just behind the 

 junction of the jugular and subclavian veins. Through this com- 

 municating branch, which is very large and has a somewhat 

 oblique course, the blood from the left jugular and subclavian 

 veins is carried across to the right Cuvierian vein, instead of 

 returning to the heart as before by the left Cuvierian vein. 

 The left Cuvierian vein, having no longer any function to 

 perform, shrinks up and becomes obliterated more or less com- 

 pletely. Portions may persist, either as fibrous cords, or as 

 venous channels of greater or less size ; and the posterior end, 

 where it opens into the sinus venosus, is said to give rise to the 

 coronary sinus. 



The pulmonary veins appear late, about the end of the fifth 

 week : they open into the left auricle, close to the interauricular 

 septum. At first there is only a single opening into the auricle, 

 but at a later stage, about the fourth month, there are two 



