544 MORPHOLOGY OF THE VENOUS SYSTEM. 



with the epigastric veins and the superficial veins of the abdominal wall ; and they sometimes 

 become much enlarged, setting up a more or less complete collateral circulation, in certain 

 diseased conditions when the branches of the portal vein within the liver are obstructed. 

 (Sappey, ' ; Memoire sur les veines portes accessoires," Journ. de 1'Anat., 1883 ; W. Braune and 

 E. H. Fen wick, " Die Venen der vorderen Rumpf wand des Menschen," 1884 ; E. Wertheimer, 

 " Recherches sur la veine ombilicale," Journ. de 1'Anat, 1886 ; P. Baumgarten, " Ueber die 

 Nabelvene des Menschen," &c., " Arbeiten aus dem pathol. anatom. Institut zu Tubingen, i, 

 1891.) 



Other communications between the portal and the general systemic veins are established 

 by means of anastomoses formed by the veins of the pancreas, duodenum, colon and rectum 

 with the parietal veins of the abdomen ; and also through the cesophageal veins and the 

 haemorrhoidal plexus. 



MORPHOLOGY OP THE VENOUS SYSTEM. 



The arrangement of the primitive venous trunks of the body is different from that of the 

 arteries, and where in the definitive state there appears to be a correspondence between the two 

 (e.g. the superior vena cava and innominate veins to the ascending aorta and the branches of 

 the arch, and the inferior vena cava to the abdominal aorta) the condition is secondary, 

 resulting from developmental modifications of the original type. The first venous stems of 

 the body are two vessels on each side, the primitive jugular vein descending ifrom the head, 

 and the cardinal vein ascending by the side of the aorta through the greater part of the 

 trunk ; the two unite to form the duct of Cuvier, which opens into the corresponding side of 

 the sinus venosus of the heart. To these must be added the vitelline or omplialo-mesentcrie 

 veins, which are the earliest of the veins to be formed, and which are converted into the 

 superior mesenteric, portal and hepatic veins (see Vol. I, p. 151), and the umbilical veins, of 

 which the right soon disappears within the body, while the left is largely developed, and 

 remains in function until the time of birth, when it also becomes obliterated, forming the 

 round ligament of the liver. 



The primitive jugular vein being joined by the subclavian vein when the upper limb makes 

 its appearance, a brachio-cephalic or innominate trunk is formed ; and the cardinal vein, under- 

 going reduction in the thoracic portion of its extent when in consequence of the development 

 of the inferior vena cava (see below) it no longer returns the blood from the lower part of the 

 body, becomes the azygos vein ; while the duct of Cuvier forms a prolongation of the brachio- 

 cephalic trunk. This symmetrical arrangement is destroyed by the occlusion of the greater 

 part of the left brachio-cephalic trunk, following upon the development of the transverse 

 jugular vein, which forms the greater part of the definitive left innominate, so that the blood 

 from both sides of the upper part of the body is collected by the trunk of the right side or 

 superior vena cava. The portion of the left trunk below the transverse jugular vein is 

 represented by a part of the left superior intercostal vein and the fibrous band in the vestigial 

 fold of the pericardium (p. 353), while the left part of the sinus venosus becomes the oblique 

 vein and coronary sinus (p. 510). At the same time the upper end of the left cardinal (azygos) 

 vein becomes obliterated, and the diminished trunk empties itself into the vein of the right 

 side through transverse prevertebral communications which are formed between the two. 

 Numerous varieties in the arrangement of the superior vena cava, innominate and azygos veins 

 are readily explicable as the result of irregularities affecting the extent and manner of the 

 occlusion of the primitive trunks (see pp. 513, 532). 



In the abdomen, the cardinal vein of each side is joined in the pelvic region, where it 

 becomes the internal iliac vein, by the primitive vein of the lower limb (sciatic), and receives 

 as it ascends branches from the Wolffian body and the abdominal wall (lumbar veins). At a 

 later period the secondary femoral vein (external iliac) and, on the development of the 

 definitive kidney, the renal vein become its principal tributaries. A shorter passage to the 

 heart is then formed by the development of a new vessel from the proximal end of the ductus 

 venosus at the back of the liver, at the spot where that canal joins the omphalo-mesenteric 

 (hepatic) veins ; this grows downwards in front of the aorta as far as the origin of the 

 superior mesenteric artery, and there bifurcates, its divisions forming a union on each side 

 with the cardinal vein where that receives the renal vein. The short trunk thus formed is 

 the hepatic portion of the inferior cava ; and as it enlarges rapidly, the continuation of the 

 cardinal vein shrinks and either disappears entirely, or remains as a communication between 

 the azygos vein and the vena cava or the renal vein (p. 530). * The change from this 

 symmetrical to the definitive asymmetrical condition takes place, as in the formation of the 



1 The ascending lumbar vein, which usually forms the beginning of the azygos vein, is a secondary 

 trunk resulting from the formation of communications between the segmental veins, and may be 

 compared to the precostal anastomoses of the segmental arteries (p. 505). 



