DEVELOPMENT OF THE VEINS. 



1041 



or cephalic part of the superior vena cava, whilst the corresponding portion of the left vein is 

 converted into the upper part of the left superior intercostal vein (Fig. 839). 



The external jugular vein is a new formation which receives for a time the cephalic vein of 

 the upper extremity ; but the cephalic vein, which is a secondary vessel, is eventually transposed 

 to the axillary vein, which is a part of the primitive upper limb vein. 



The Posterior Cardinal Veins, the Subcardinal Veins, and the Inferior Vena Cava. 

 The posterior cardinal veins appear later than the anterior cardinal veins and they terminate 

 cranialwards in the ducts of Cuvier. They lie dorsal to the Wolman bodies and they 

 become connected with each other, dorsal to the descending aorta, by numerous transverse 

 anastomoses. 



The subcardinal veins appear later than the posterior cardinals. They lie along the ventral 

 borders of the Wolman bodies and they are connected not only by dorso-ventral anastomoses with 

 the posterior cardinal veins, but also by transverse anastomoses with one another. The majority 

 of both sets of anastomosing vessels ultimately disappear, but two remain ; one which joins the 

 right subcardinal to the right posterior cardinal, at the level of the right renal vein ; and one 

 which connects the subcardinal veins together, across the ventral surface of the aorta, at the 

 same level. 



After a time an anastomosis is formed between the right subcardinal vein and the cephalic 

 end of the right vena revehens, dorsal to the liver, by the caudal outgrowth of an anastomosing 



Superior sagittal sinus 



Inferior sagittal sinus 

 "* Internal cerebral veins 



s verse sinus, 

 irizontal part 

 = upper part of | 



middle stem 



Sigmoid part of 

 transverse sinus 



linal part of sigmoid portion" 

 of trans verse jsinus = posterior stem 



FIG. 838. DIAGRAM OF 



Straight sinus 

 'Great cerebral vein 



~- Remnant of anterior stem 



thalmic vein 



Semilunar ganglion 

 Inferior petrosal sinus 



N Superior petrosal sinus = lower part of middle stem 



Internal jugular vein 



VENOUS SINUSES. (Only one transverse sinus is shown). 



i offset from the right vena revehens. As soon as the anastomosis is completed the blood from 

 the caudal portion of the body and from the lower limbs is short-circuited to the heart, and 

 extensive changes occur in the primitive posterior cardinal veins, into which in the meantime 

 . the veins from the lower extremities have opened. 



The Posterior Cardinal Veins. The right posterior cardinal vein, cephalwards of the right 



i renal vein, becomes the vena azygos. Betwe'en the right renal vein and the entrance of the 



lower limb vein it forms the caudal or lower part of the inferior vena cava and the right common 



iliac vein ; the remainder of the right posterior cardinal vein becomes the right hypogastric vein. 



Two of the transverse anastomoses between the posterior cardinal veins form the transverse 

 parts of the hemiazygos and accessory hemiazygos veins ; others become converted into those parts 

 : of the left lumbar veins which lie dorsal to the aorta, and one, which lies opposite the fifth lumbar 

 somite, becomes the greater part of the left common iliac vein. 



Portions of the left posterior cardinal vein remain as the left hypogastric vein, the 

 hemiazygos and the accessory hemiazygos veins and the lower part of the left superior intercostal 

 vein. 



The tributaries of the right posterior cardinal vein become the right intercostal, subcostal, 

 md the right lumbar veins. The right renal vein is also one of its tributaries. The tributaries 

 3f the left posterior cardinal become the left intercostal and subcostal veins, and they form those 

 3arts of the left lumbar veins which lie to the left of the vertebral column, and the corresponding 

 aart of the left renal vein (Fig. 839). 



The Subcardinal Veins. The only important parts of the subcardinal system which remain 

 n the adult are a portion of the right subcardinal vein, one of its anastomoses with the right 

 oosterior cardinal vein, and a transverse anastomosis between the subcardinal veins. The last 



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