694 



THE BLOOD-VASCULAR SYSTEM 



tive cases of abnormalities of the vena cava inferior have recently been published by v. Alten 

 and by Neuberger (see References). Both articles contain bibUographies. The chief varia- 

 tions are as follows: — 



(1) The inferior vena cava, in cases of transposition of the viscera, may lie on the left side 

 of the aorta. (2) Without transposition it may also lie to the left of the aorta, crossing to the 

 right to gain the caval opening immediately below the diaphragm, or after receiving the left 

 renal vein. (3) It may be double, the left cava than usually passing across the aorta into the 

 right after receiving the left renal vein. A communication between the right and left veins in 

 the position of the normal left common iliac vein may or may not then exist. (4) The inferior 

 vena cava may be absent, the blood from the lower extremities passing by a large vein in the 

 position of the ascending lumbar and azygos veins through the diaphragm to open into the 

 superior vena cava. The hepatic veins then open directly into the right atrium through the 

 normal caval opening in the diaphragm. (5) The inferior vena cava may receive the left sper- 

 matic vein. (6) It may receive a left accessory renal vein passing behind the aorta, and into 

 this the usual tributaries of the left renal vein may open. (7) It may receive several accessory 

 renal veins; as many as seven on each side have been met with. (8) The lumbar veins may 

 enter it on one or both sides as a common trunk. 



The variations in the veins of the lower extremity are for the most part unimportant. 

 They have been mentioned in the description of the corresponding veins. 



3. THE PORTAL SYSTEM OF VEINS 



The portal system arises by transformations in the vitelline and umbilical veins. The 

 proximal ends of the vitelline veins, where they lie between the umbilicals, are early enveloped 

 in, and invaded by, the growing liver. The columns of liver cells, while not penetrating the 

 endothelium, subject the vitelline veins to a process of fenestration by which the original channels 

 are subdivided into innumerable smaller vessels or sinusoids. The sinusoids arising from the 

 two vitelline veins intercommunicate to form one continuous network in which the vessels are 

 larger in the afferent (portal) and efferent (hepatic) areas than in the intermediate zone. 



Fig. 545. — Semidiagrammatic Reconstrtjctions of the Veins of the Liver, Ventral 

 Aspect (Mall). A, Embryo of 4.5 mm. Long; B, 4 mm. (more advanced than A); C, 7 mm. 

 d.v., ductus venosus; L, intestine; L., liver; m., superior mesenteric (continued as portal) 



vein; r.a., ramus angularis; r. a'., right branch of portal vein; r.h.d., right hepatic vein; 



r.h.s., left hepatic vein; r.u., recessus umbilicalis; u.v., left umbilical vein (the right 



umbilical vein is not labelled); v.o.m., vitelline veins. 



The two umbilical veins now form communications with the portal area of the sinusoidal 

 network and eventually lose their original connections with the sinus venosus (fig. 545). The 

 fate of the umbilical veins differs on the two sides; the right degenerates, from the sinus venosus 

 to the common umbilical vein, and leaves the left to receive all the blood flowing from the pla- 

 centa. The left, having lost its connection with the sinus venosus, discharges its blood partly 

 into the portal sinusoidal zone, and partly, by means of the newly formed direct channel, the 

 ductus venosus, into the right vitelline (fig. 545). 



The hepatic end of the right vitelline vein enlarges considerably, for the left vitelline loses 

 its original connection with the sinus venosus. It transmits blood both from the sinusoids 

 and from the ductus venosus to the sinus venosus, and is called the common hepatic. 



The vitelline veins are not only connected within the liver, but their distal ends become 

 united upon the yolk-stalk to form a single trunk. A third communication between them is 

 effected by a transverse vessel passing dorsal to the duodenum. The portion of the right 

 vitelline below the transverse vessel disappears, as does the portion of the left between it and 

 the liver. A tortuous vitelline vein is thus produced which enters the liver by passing dorsal 

 to the intest.ine from loft to right. This vessel is joined, to the loft of the intestine, by the 

 superior mesenteric vein and, dorsal to it, by the splenic. When the portion of the vitelline 

 below the termination of tho superior mesenteric finally disappears the vessel extending from 

 the splenic vein to the liver becomes the portal vein of the adult. 



I Important variations of tho portal system are rarely found in the adult. The mechanism 

 of anomalies found in the cmbrj'o have been investigated by Begg (Amcr. Jour. Anat., Vol. 13). 



I 



