FORMATION OF THE GEEAT VEIKS, 



797 



part of its extent it becomes the external jugiilar vein : and near its lower end 

 it receives small brandies, which, grow to be the external jugular and subclavian 



Fig. 594. — Diagrams illustrating 



THE DEVELOPMENT OF THE GrEAT 



Veins (after KiJlliker). 



A, plan of the principal veins of the 

 fretus of about four weeks, or soon after 

 the first formation of the vessels of the 

 hver and the vena cava inferior. 



B, veins of the liver at a somewhat 

 earlier period. 



C, principal veins of the foetus at the 

 time of the first estabhshment of the 

 placental circulation. 



D, veins of the liver at the same 

 period. 



dc, the right and left ducts of Cuvier ; 

 ca, the right and left cardinal veins ; 

 j. j,ihe jugular veins ; s, the subclavian 

 veins ; az, the azygos vein ; u, the um- 

 bilical or left umbilical vein ; «', in B, 

 the temporary right umbilical vein ; o, 

 the omphalo-meseuteric vein ; o', the 

 right omphalo-mesenteric vein ; m, the 

 mesenteric veins ; p, the jjortal vein ; 

 p', p', the vente advehentes ; I, the duc- 

 tus venosus ; V, I', the hepatic veins ; 

 c'l, vena cava inferior ; il, the division 

 of the vena cava inferior into common 

 iliac veins ; cr, the external iliac or 

 crural veins ; h, the hypogastric or in- 

 ternal iliac veins, in the line of continua- 

 tion of the jirimitive cardinal veins. 



In C, li, in dotted lines, the trans- 

 verse branch of communication between 

 the jugular veins which forms the left 

 innominate vein ; ri, the right innomi- 

 nate vein ; ca, the remains of the left 

 cardinal vein by whish the superior 

 intercostal veins fall into the left in- 

 nominate vein ; above lo, the obliquely 

 crossing vein by which the hemiazygos 

 joins the azygos vein. 



veins. The cardinal veins are the primitive vessels which return the blood from 

 the Wolffian bodies, the vertebral column, and the parietes of the trunk. The 

 inferior vena cava is a vessel of later development, wliich opens into the trunk 

 of the umbilical and omphalo-mesenteric veins, above the vense heijaticas reve- 

 hentes. The iliac veins, which unite to form the inferior vena cava, communi- 

 cate with the cardinal veins. The inferior extremities of the cardinal veins are 

 persistent as the internal iliac veins. Above the iliac veins the cardinal veins 

 are obliterated in a considerable part of theii' course ; their upper portions then 

 become continuous with two new vessels, fhe j;osfe?-ipr vertchral veins of Rathke, 

 which receive the lumbar and intercostal twigi-'. 



As development proceeds, the direction of the ducts of Cuvier is altered by the 

 descent of the heart from the cervical into the thoracic region, and becomes 

 continuous with that of the primitive jugular veins. A communicating branch 

 makes its appearance, directed transversely from the junction of the Jeft sub- 

 clavian and jugular veins, downwards, and across the middle line to the rig-ht 

 jugular ; and further down in the dorsal region between the posterior verteljral 

 veras a communicating branch passes obliquely across the middle line from right 



