THE THORAX 1005 



clavian artery, so that the subclavian artery is originally a lateral branch of 

 the vertebral artery. As development proceeds, however, the subclavian 

 artery increases in size, and greatly exceeds the vertebral artery, the latter 

 vessel being now regarded as a branch of the subclavian, whereas originally 

 it is the parent trunk. 



The thoracic and abdominal segmental arteries have been already referred 

 to in connection with the dorsal aortae. 



Development of the Principal Veins. 



The primitive veins form tv:6 groups. One group returns the blood from 

 the yolk-sac and the placenta; and the other group returns the blood from 

 the head and neck, anterior limbs, body-wall. Wolffian bodies, and posterior 

 limbs. The first group comprises: (i) The vitelline veins, in connection 

 with which the portal vein is developed; and (2) the umbihcal or allantoic 

 veins. The second group consists of (i) the anterior cardinal veins; (2) the 

 posterior cardinal veins ; and (3) the subcardinal or supracardinal veins. The 

 veins of each group are arranged in pairs, right and left, as follows: 



Two Vitelline Veins. Two Anterior Cardinal Veins. 



Two Umbilical Allantoic Veins. Two Posterior Cardinal Veins. 



Two Subcardinal or Supracardinal Veins. 



Vitelline or Omphalo-Mesenteric Veins and Vena Port». — These veins are 

 developed first, and they return the blood from the yolk-sac. They enter 

 the lody of the embrj'o along the vitelUne duct, and finally open into the 

 sinus venosus, after traversing the septum transversum. Within the body 

 they ascend, parallel with each other, at first in front of, and subsequently 

 on either side of, the duodenal portion of the primitive intestinal tube. In 

 the latter region, on the caudal side of the hepatic bud, they become con- 

 nected by three transverse anastomotic vessels, tr^vo of which he across the 

 ventral aspect of the gut, and one being placed on its dorsal aspect. The first 

 or lowest anastomotic vessel hes on the ventral aspect of the gut; the second 

 or middle vessel is dorsal to the gut; and the third or highest, like the first 

 or lowest, is ventral to the gut. 



By means of these three anastomotic vessels two venous rings — lower, or 

 caudal, and upper, or cephalic — are formed around the duodenal portion of 

 the primitive intestinal tube, these rings constituting the sinus annularis. 

 During their formation the two divisions of the hver-bud are breaking up 

 into hepatic cylinders, and these are giving off secondary cyUnders. Owing 

 to these hepatic developments the vitelline veins cease to communicate 

 directly with the sinus venosus. The portions of the vitelline veins above 

 the upper duodenal venous ring (shown in Fig. 417 to proceed from its sides) 

 become surrounded by the hepatic cylinders, and invaded by the secondary 

 cylinders. In this manner these portions of the vitelline veins are freely 

 subdivided into blood-channels, which are known as sinusoids (Minot). These 

 sinusoids form a netsvork which occupies the meshes of the netvvork formed 

 by the branches of the hepatic cylinders. The veins which convey blood 

 from the upper duodenal ring to the hepatic sinusoids are now known as the 

 vencs hepaticcE advehentcs, and they become the right and left divisions of the 

 portal vein. The veins which carr>' the blood from the hepatic sinusoids to 

 the sinus venosus are known as the vence hepatic^ revehentes, and they become 

 the hepatic veins. 



Trunk of the Portal Vein. — The portions of the two vitelline veins which 

 ascend in front of the primitive duodenum he close together and parallel 

 with each other. These portions fuse for a short distance, and form a single 

 venous stem, which opens into the first, or lowest, ventral anastomotic vessel, 

 or, in other wordS; into the lower part of the lower duodenal venous ring. 

 This short stem receives the veins of the primitive intestinal tube, and it 

 forms the root of the portal vein. The primitive portal vein, therefore, receives 



