1038 



THE VASCULAK SYSTEM. 



the latter becomes the left vena advehens, persists until birth and, after the disappearance of the 

 right lateral umbilical vein, it conveys the blood from the placenta to the liver, where part of the 

 placental blood passes into the left vena advehens and so through the left vena revehens to the 

 inferior vena cava, and part passes into the ductus venosus, by which it reaches that portion of 

 the cranial part of the right vena revehens which becomes the cranial or upper end of the 

 permanent inferior vena cava. 



After birth when the placental circulation ceases the left lateral umbilical vein becomes the 

 ligamentum teres of the liver. 



Middle plexus 



Posterior plexus 



Anterior plexus 



Posterior stem 

 Otic vesicle 



'"^ Anterior stem 



' x Ophthalmic vein 

 Anterior cardinal vein j Semilunar ganglion 



Middle stem 

 Primary head vein 

 FIG. 834. DIAGRAM OP THE PRIMARY HEAD VEIN AND ITS TRIBUTARIES. (After Streeter.) 



The Ductus Venosus. The ductus venosus is developed as the left lateral umbilical vein 

 loses its direct connexion with the liver and becomes united to the left vena advehens. It is 

 formed from the sinusoidal spaces of the rudimentary liver and connects the commencement of 

 the left vena advehens with the cranial part of the right vena revehens. It forms the more 

 direct channel by which blood from the placenta is passed to the heart through that part of the 

 right vena revehens which becomes the upper end of the inferior vena cava. After birth it 

 converted into the fibrous ligamentum venosum, which connects the left branch of the port 

 vein with the upper end of the inferior vena cava. 



Middle and anterior plexuses 



Posterior plexus 



Upper part of anterior stem 



Middle stem 



_! Lower part of anterior stem 



Ophthalmic vein 



Internal jugular vein 



V 



Parts of primary head vein 

 FIG. 835. DIAGRAM or THK HEAD VEINS OF A 21 MM. EMBRYO. (After Streeter.) 



The Anterior Cardinal Veins. The anterior cardinal veins are the veins by which t. 

 blood is returned to the heart from the head and neck and, ultimately, from the upper extn 

 ties also, although the primitive veins of the upper extremities are, in the first place, trib 

 of the posterior cardinal veins. 



Each anterior cardinal vein may be separated into two parts the intra- cranial and 

 extra- cranial 



In the majority of vertebrates the portion of each anterior cardinal vein which is 

 in the head region, and which afterwards becomes intra-cranial, lies to the medial side c 

 otic vesicle and the 5th, 7th, 8th, 9th, 10th, and llth cerebral nerves. 



