184 THE CIRCULATORY APPARATUS 



the muscular walls of the ventricles, the gradual 

 closure and vibrations of the mitral and tricuspid 

 valves, and the sudden pressure of the apex against 

 the chest wall. The second sound is supposed to be 

 due to the sudden closure and vibrations of the semi- 

 lunar valves in the aorta and pulmonary artery, 

 following the ventricular systole or contraction; also 

 the sound is added to by the whirling of the column 

 of blood against those closed valves at the beginning 

 of diastole or relaxation of the ventricles. 



The Fetal Circulation. The fetus is nourished by 

 the blood from the placenta (afterbirth). The blood 

 is conveyed from the placenta to the fetus by the 

 umbilical vein. This vein enters the umbilicus and 

 passes upward along the upper free margin of the 

 suspensory ligament of the liver to the under surface 

 of the liver. The blood after nourishing the organ 

 by two or three branches, finally reaches the inferior 

 vena cava by way of the hepatic veins, and the ductus 

 venosus, which runs from the transverse fissure of 

 the liver to open into the hepatic veins just before 

 they open into the inferior vena cava. The superior 

 and inferior vena cava open into the right auricle of 

 the heart. 



The course of the blood through the chambers of 

 the fetal heart differs from that observed in the adult, 

 viz., the blood from the inferior cava passes into the 

 right auricle and then is directed by the Eustachian 

 valve to the left auricle of the heart through the 

 foramen ovale. In the left auricle the blood from the 

 right auricle becomes mixed with a small quantity 

 of blood returned from the lungs by the pulmonary 

 veins. This blood then passes into the left ventricle 

 (as seen in the adult) and then into the aorta, by which 

 vessel it is distributed almost entirely to the head and 

 upper extremities. The blood is returned from the 

 head and upper extremities by the veins, w r hich 

 ultimately drain into the superior vena cava, that 



