DEVELOPMENT OF THE VESSELS. 1C5 



The descending aorta appears to be the remnant of the artery formed by the 

 union of the two primitive aortse. The omphalo-mesenteric arteries, which 

 spring from these latter, all disappear except one, which remains as the superior 

 mesenteric artery. The umbilical arteries are at first the terminations of the 

 two primitive aortae, but when these vessels are united into one, the umbilical 

 arteries appear as branches, and the aorta itself ends in a caudal prolongation, 

 which afterwards becomes the middle sacral. The common and internal iliac 

 arteries are the only permanent remains of the umbilical arteries (see Internal 

 Iliac Arteries.) 



Veins. The primitive venous circulation has been described above, the two 

 omphalo-mesenteric veins opening by a common trunk into the lower end of 

 the tube, which represents the heart. The next state of the venous circulation 

 is, that at about four weeks there is found a single vein lying behind the in- 

 testinal cavity (not in front of it, as the temporary omphalo-mesenteric veins 

 do), and receiving the trunk vein from the intestine (mesenteric). Two um- 

 bilical veins are early formed, and open together into the common trunk of 

 the omphalo-mesenteric vein. They receive branches from the allantois and 

 anterior surface of the embryo. The right vein soon disappears ; the left um- 

 bilical vein, on the contrary, grows till it becomes the trunk vessel into which 

 the omphalo-mesenteric vein and its mesenteric branch appear to open. Next 

 the liver begins to be formed around the umbilical vein, and then this vein 

 sends branches into that gland (afferent veins) which afterwards become the 

 portal veins in the interior of the liver, and which give origin to other veins 

 (efferent), which return the blood from the liver, and form afterwards the 

 hepatic veins. The portion of the umbilical vein between the giving off of 

 the future portal vessels and the reception of the hepatic, forms the ductus 

 venosus. The mesenteric vein communicates at first with the omphalo-meseu- 

 teric ; when the veins of the liver are formed, the omphalo-mesenteric is trans- 

 ferred from the umbilical vein to the right afferent hepatic. A portion of it 

 persists and forms the trunk of the portal vein. 



The systemic veins are developed from four trunk veins, two on either side, 

 above and below, which appear before the formation of the allantois or the 

 umbilical vessels. These unite into one canal on either side (canal of Cuvier), 

 which open into the common trunk of the omphalo-mesenteric veins, and so 

 into the auricular portion of the rudimentary heart. These four primitive 

 veins lie, two of them in front, the anterior cardinal, or jugular veins, and the 

 other two behind, the posterior cardinal veins. As the umbilical vein increases, 

 and the omphalo-mesenteric diminishes in volume, the sinuses of Cuvier are 

 transferred to the former vein, and when the inferior cava is formed and the 

 umbilical vein becomes merely its tributary, the sinuses of Cuvier open into 

 the inferior vena cava. At a later period the portion of the vena cava inferior, 

 between the opening of the sinuses of Cuvier and the auricle, disappears, and 

 then the auricle receives three veins viz., the inferior cava, and the two sinuses 

 of Cuvier, which are now called right and left superior vena cava (Fig. 66). 

 The superior cardinal, or jugular veins, which form the upper branches of the 

 sinuses of Cuvier on either side, unite about the second month by a transverse 

 anastomosing branch. The left superior vena cava assumes an oblique posi- 

 tion, and empties itself into the lower and left end of the auricle. Finally, its 

 trunk disappears, while its orifice is transformed into the coronary sinus, in 

 which the great cardiac vein opens. The right sinus of Cuvier, or superior 

 vena cava, persists ; the transverse anastomosing branch between the two jugu- 

 lars becomes the left innominate vein, and the end of the right jugular the 

 right innominate. The venous circulation in the lower part of the embryo is 

 at first carried on by the inferior cardinal veins, which return the blood from 

 the Wolffian bodies, and receives branches corresponding to the intercostal, 

 lumbar, and crural veins. 



