520 SANGUIFEROUS SYSTEM. 



left auriculo-ventricular orifice, into the left ventricle (C. a.) 

 of the heart, by which it is propelled into the aorta (C. h) 

 and into all the branches (C. i. k. I.) ascending from its arch, 

 to nourish and develope the brain, the organs of the senses, 

 and all the important anterior parts of the body, so early 

 and largely developed in the foetus. 



The blood thus rendered venous by affording nutri- 

 ment to the larger anterior half of the body, is returned, 

 in an impure state, by the superior vena cava (C. .), 

 to the right auricle (C. e.) By the right auriculo-ventri- 

 cular orifice it passes directly into the right ventricle 

 (C. o.), which propels it into the pulmonary artery (C. r.) 9 

 the ductus arteriosus (C. /.), and the descending aorta 

 (C. m.), to afford a scanty nutriment to the small pos- 

 terior parts of the trunk and the limbs, and to proceed in 

 mass through the large umbilical arteries (142. B. u. v.) for 

 fresh oxygenation in the placenta. A small portion only of 

 this blood is yet sent by the right (C. s. B. h.) and left (C. u. 

 B. i.) branches of the pulmonary artery (C. r. B. g.}, to the 

 rudimentary and ineffective lungs; and the external and 

 internal iliacs are yet but small branches of the great umbilical 

 arteries which here form the bifurcation of the aortal trunk. 

 Both ventricles thus combine, to propel the vital fluid through 

 the system of the foetus, and through the long and tortuous 

 windings of the umbilical and placental vessels, and a mixed 

 blood circulates through the arteries of the body, as through 

 the body of a cold-blooded reptile. It is only by the subse- 

 quent closing of the umbilical arteries and vein, and the obli- 

 teration of the ductus venosus y the foramen ovale, and the 

 ductus arteriosus, that man's circulating system is raised to 

 the mammiferous type, after a proteus-like career through 

 every inferior form presented by this system throughout the 

 animal kingdom. 



