278 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



Changes at Birth.— At birth the umbilical vessels are ruptured and the lungs 

 become functional. The umbilical arteries and veins, no longer used, contract 



Fig. 286. — Diagrammatic outline of the organs of circulation in the fetus of six months (Allen 

 Thomson).' RA, Right atrium of the heart; RV, right ventricle; LA, left atrium; Ev, valve of inf. vena 

 cava; LV, left ventricle; L, liver; K, left kidney; I, small intestine; a, arch of the aorta; a', its dorsal 

 part; a", lower end; lies, superior vena cava; vci, inferior vena cava where it joins the right atrium; vci' 

 its lower end; s, subclavian vessels; J, right jugular vein; c, common carotid arteries; four curved dotted 

 arrow lines are carried through the aortic and pulmonary opening and the atrio-ventricular orifices; da, 

 opposite to the one passing through the pulmonary artery, marks the place of the ductus arteriosus; 

 a similar arrow line is shown passing from the vena cava inferior through the fossa ovalis of the right 

 atrium and the foramen ovale into the left atrium; hv, the hepatic veins; vp, vena portae; .v to vci, the duc- 

 tus venosus; uv, umbilical vein; ua, umbilical arteries; tec, umbiUcal cord cut short; i, ;', iliac vessels. 



and their lumina are obliterated by the thickening of the inner coat (tunica in- 

 tima). The lumen of the umbilical artery is occluded after four days, that of 



' In this diagram the arteries are conventionally colored red and the veins blue, but these colors 

 are not intended to indicate the nature of the blood conveyed by the respective vessels. 



