DEVELOPMENT OF THE PRINCIPAL VEINS. 



153 



T\\Q primitive jugular vein receives the blood from the cranial cavity by channels 

 in front of the ear, which are subsequently obliterated : in the greater part of its 

 extent it becomes the external jugular vein ; and near its lower end it receives small 

 branches, which grow to be the internal jugular and subclavian veins (fig. 185). 

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

 Wolffian bodies, the vertebral column, and the parietcs of the trunk. The inferior 

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

 umbilical and vitelline veins, above the venae hepaticse revehentes. The iliac veins, 



Fig. 185. DIAGRAM OP THE CONDITION OP 



THE GREAT VEINS IN THE EMBRYO, AND 

 OP THEIR TRANSFORMATION INTO THE 



PERMANENT VEINS. (After Kolliker. ) 



j, jugular veins uniting with the subcla- 

 vian, s; c.a, cardinal veins becoming the 

 azygos as in B ; d. c, ducts of Cuvier formed 

 by the union in B of jugular and cardinal, 

 and becoming the superior vena cava on the 

 right side, but disappearing on the left side ; 

 ca f , part of left cardinal vein which disap- 

 pears ; ci, vena cava inferior ; I, hepatic 

 veins and ductus venosus ; il, common iliac 

 veins ; cr, external iliacs ; h, hypogastric 

 becoming the internal iliacs. 



which unite to form the inferior 

 vena cava, communicate with the 

 cardinal veins. The inferior extre- 

 mities of the cardinal veins are per- 

 sistent as the internal iliac veins. 

 Above the iliac veins the cardinal 

 veins are obliterated in a consider- 

 able part of their course ; the upper 

 portions then become continuous 

 with two new vessels, the posterior 

 vertebral veins of Eathke, which 

 receive the lumbar and intercostal 

 twigs. 



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

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

 same as that of the primitive jugular veins. A communicating branch makes its 

 appearance, directed transversely from the junction of the left subclavian and 

 jugular veins, across the middle line to the right jugular ; and further down 

 in the thoracic region between the posterior vertebral veins, a communicating 

 branch passes obliquely across the middle line from right to left. The communi- 

 cating branch between the primitive jugular veins is converted into the left brachio- 

 cephalic or innominate vein. The portion of vessel between the right subclavian 

 vein and the termination of the communicating branch becomes the right brachio- 

 cephalic vein. The portion of the primitive jugular vein below the communicating 

 vein, together with the right duct of Cuvier, forms the vena cava superior, while 

 the cardinal vein opening into it is the extremity of the great vena azygos. On the 

 left side, the portion of the primitive jugular vein placed below the communicating 

 branch, and the cardinal and posterior vertebral veins, together with the cross 

 branch between the two posterior vertebral veins, are converted into the left 

 superior intercostal and left superior and inferior azygos veins. The variability 

 in the adult arrangement of these vessels depends on the different extent to which 

 the originally continuous vessels are developed or atrophied at one uoint or another. 



or 



