DEVELOPMENT OF THE VASCULAR SYSTEM. 



993 



The right of these, however, soon atrophies. As soon as the allantois is formed, 

 both allantoic or umbilical veins unite to form the truncus venosus (i, u u^. 

 At first the omphalomesenteric veins are larger than the umbilical veins: later, 

 the conditions are reversed, and also the right umbilical vein atrophies. As soon 

 as the veins of the trunk itself are formed, the inferior cava also empties into 

 the truncus venosus (2 , C i). Gradually the umbilical vein becomes the chief 

 path (2 , tt t ) , to which the small omphalomesenteric vein (2 , omj sends but little 

 blood. 



The umbilical and omphalomesenteric veins pass in part directly beneath 

 the liver to the heart; in part, however, they send also branches, carrying arterial 

 blood, into the liver, which grows around the vessels from above the venae 

 advehentes (2 and 3 , a) . The blood from the latter passes back through other 



Of/I 



om, 



1. 



u u 



FIG 



387. Development of the Veins of the First and the Second Circulation, and of the Portal System: H, heart; 

 R, right, L left side of the body, om, right omphalomesenteric vein, om\, left omphalomesenteric vein; , 

 right umbilical vein; Wj, left umbilical vein; Ci, inferior vena cava; a, venae advehentes; r, vena? reve- 

 hentes; D, intestine; m, mesenteric vein; 4 /, splenic vein; 2 /, liver. (Diagrammatic.) 



veins (venae revehentes, 2 and 3, r), which again unite with the main trunk of 

 the umbilical veins at the blunt border of the liver. In the liver, the umbilical 

 vein (3, Uj) anastomoses with the omphalomesenteric vein (3, owj. 



.With the development of the bowel (3, D ) the mesenteric vein (wi) empties 

 into the omphalomesenteric; as does also the splenic vein (4, /) with the develop- 

 ment of the spleen. When, later, the omphalomesenteric vein atrophies (4, om^), 

 the mesenteric vein is the sole trunk of these previously united vessels. It is, 

 therefore, the vein that unites with the umbilical vein in the liver, and it thus 

 represents the trunk of the portal vein. When, finally, at birth the umbilical 

 vein atrophies (4, %) , the mesenteric vein alone remains as the portal vein. This 

 must, however, send all its blood through the liver, as the ductus venosus of 

 Arantius (4, Da) becomes obliterated. In this way the portal circulation is com- 

 pleted. 



DEVELOPMENT OF THE ALIMENTARY CANAL. 



The primitive intestine is originally a straight tube, passing from the cranial 

 to the caudal extremity. The omphalomesenteric duct is inserted at a point 

 that corresponds later to the lower part of the ileum. Here the tube in the fourth 

 week makes a slight bend toward the navel (Fig. 388, /). It has already been 

 pointed out that the duct later is obliterated (intestinal navel) and is finally 

 detached as a thread from the intestinal tube; it is still discernible in the third 

 month. In rare cases a short blind tube joined to the bowel persists as a vestige 

 of the incompletely obliterated duct. This is the so-called true intestinal diver- 

 ticulum: occasionally a cord (the obliterated omphalomesenteric vessels) passes 

 from it to the navel; in rare cases the duct may remain open through the navel 

 even after birth, so that a congenital fistula of the ileum results; or finally the 

 diverticulum may be the seat of cyst-formation. In human embryos four weeks 

 old, His was able to differentiate the mouth, the pharynx, the esophagus, the 

 stomach, the duodenum, the mesenteric intestine, the end-gut and the cloaca. 



63 



