930 



HUMAN ANATOMY. 



Fig. 780. 



the pulmonary artery and towards the lungs. Being uninflated these organs can 

 appropriate only a small part of the entire volume of blood brought by the pulmonary 

 artery, hence the necessity of a second by-pass, the ductus arteriosus, or ductus 

 Botalli, that extends from the beginning of the left pulmonary artery to the 

 adjacent aorta and represents the still pervious distal portion of the last aortic arch 



on the left side (page 847). By means 

 of the ductus arteriosus, the venous blood 

 returned from the head and upper 

 extremities is poured into the great 

 descending trunk, the aorta, and carried 

 to the abdominal viscera and the lower 

 extremities. On reaching the bifurcation 

 of the common iliac arteries, the blood- 

 stream divides, that part going into the 

 internal iliacs being of much greater 

 importance, so far as the general nutri- 

 tion of the foetus is concerned, since it 

 is carried by the continuations of these 

 vessels— the hypogastrics and umbilical 

 arteries — to the placenta, to be once more 

 purified and again returned to the foetus 

 by the umbilical vein. 



From the foregoing sketch of the 

 foetal circulation it is evident that, with 

 the exception of the umbilical vein, no 

 vessel within the foetus conveys strictly 

 arterial or fully oxygenated blood, since 

 on entering the inferior cava the pure 

 blood is mixed with the venous returning 

 from the lower half of the body. It is 

 further evident that the blood distributed 

 to the head and upper extremities is less 

 contaminated than that passing to the 

 lower half of the body from branches of 

 the aorta given off after junction with 

 the venous stream conveyed by the 

 ductus arteriosus. It may be borne in 

 mind that the umbilical vein and the 

 ductus venosus carry arterial blood and 

 the pulmonary artery and the ductus 

 arteriosus purely venous blood, the aorta 

 distributing mixed. Upon the assumption 

 of the respiratory function at birth, the 

 three anatomical structures peculiar to 

 the foetal circulation — the ductus venosus, 

 the foramen ovale, and the ductus arteri- 

 osus — become useless and soon undergo 

 occlusion and atrophy, the two former 

 ducts being represented by the fibrous cords seen on the posterior surface of the liver 

 and terminal part of the aortic arch respectively. Closure of the foramen ovale 

 proceeds more slowly, a week or more being usually consumed in effecting 

 obliteration of the opening ; indeed, in a large proportion of individuals complete 

 closure never occurs ( page 695). 



Diagram of foetal circulation shortly before birth : 

 courseof blood is indicated by arrows. P, placenta; UA, 

 UV, umbilical arteries and vein; U, umbilicus; DV, 

 ductus venosus; IVC, inferior vena cava; PV, portal 

 vein ; HV, hepatic veins ; RV, LV, right and left 

 ventricle; PA, pulmonary artery; DA, ductus arteri- 

 osus ; SVC, superior vena cava ; AA, abdominal aorta ; 

 HA, hypogastric arteries (internal iliac); EIA, ex- 

 ternal iliac arteries ; 1, intestine; L, lungs; K, kidney. 



