CHAP. XXViii.] FORCES WHICH CIRCULATE THE BLOOD. 349 



into the inferior vena cava, just before that vessel communicates 

 with the heart. This vessel is the ductus venosus a continuation of 

 the trunk of the umbilical vein through which some of the blood 

 returning from the placenta, passes directly to the inferior vena cava, 

 and to the right auricle of the heart, without traversing the liver. 



The blood thus received from the inferior vena cava (being that 

 from the body below the diaphragm), and also from the placenta, 

 does not pass into the right ventricle. A very interesting piece of 

 mechanism obstructs its passage in that direction, and favours its 

 flow across the right auricle through the foramen ovale, now freely 

 open in the septum, into the left auricle. This is the Eustachian 

 valve, which is situated between the inferior vena cava and the 

 right auriculo-ventricular opening, and being connected with the 

 anterior and inferior part of the annulus ovalis, it brings the fora- 

 men ovale into very close connexion with the inferior vena cava, 

 and forms an imperfect septum towards the auriculo-ventricular 

 opening, quite sufficient, however, to impede the flow of blood in 

 the downward direction. 



Arrived in the left auricle, the blood is transmitted thence to the 

 left ventricle, and from this latter cavity through the arch of the 

 aorta to the head, neck, and upper extremities, whence it is re- 

 turned by the venae innominatse and by the superior vena cava to 

 the right auricle, which transmits it to the right ventricle. This 

 latter ventricle propels it into the trunk of the pulmonary artery, 

 which in the foetus, divides into three vessels, not into two, as in 

 the adult. These are the two pulmonary arteries which separate 

 from the trunk at right angles, one for each lung, and between 

 them, following the direction of the parent trunk, a large vessel, 

 nearly as large as the pulmonary artery itself, which forms a direct 

 anastomosis with the aorta, just below its arch. This is the ductus 

 arteriosus, through which the blood is transmitted directly from 

 the right ventricle to the commencement of the abdominal aorta. 



Of the Forces by which the Blood is circulated. The principal 

 force by which the blood is moved throughout the vascular system, 

 and returned to the heart, is that which is generated by the con- 

 traction of the left ventricle, or what is commonly called the vis a 

 tergo of the heart. 



The force with which the heart propels the blood into the arte- 

 rial system has been variously estimated. Valentin considers that 

 the left ventricle exerts a force equal to one-fiftieth of the weight 

 of the body, and taking the muscular power of the right ventricle 



