BLOOD SYSTEM. 



377 



the left ventricle, the blood is thrown into the great 

 arterial trunk, called the aorta. This forms an arch — 

 the aortic arch — from the top of which go branches to 

 the upper part of the trunk, arms, 

 and head, while the main part 

 turns downward, running along 

 the backbone, branching and re- 

 branching to supply the viscera 

 and lower trunk and limbs. It 

 left the heart bright blood ; in 

 the tissues it gives up O and 

 takes CO s and becomes dark 

 blood, and in this condition is 

 brought back— that from above 

 by the vena cava descendens, 

 that from below by the vena cava 

 ascendens — to the right auricle. 

 The right auricle contracts and 

 throws it into the right ventri- 

 cle, which in turn throws it 

 through the pulmonic artery, to 

 be distributed through the capil- 

 laries of the lungs, where it dis- 

 charges its C0 2 and retakes O, 

 and becomes again bright blood, 

 left auricle and to the left ventricle, to commence again 

 another round. 



Such is a most generalized formula. Fig. 259 shows 

 it still very diagrammatically, but yet a little more in 

 detail, especially as to the great branches supplying the 

 liver, spleen, the intestines, and the kidneys, and the 

 corresponding veins to make up the vena cava ascendens. 

 Mark here the portal vein already spoken of (page 328) 

 and its singularity. 



In both these diagrams the arterial trunks are on one 



ex. 



Fig. 258. — Diagram showing 

 general course of the cir- 

 culation : upex, upper ex- 

 tremities and head ; lex, 

 lower extremities ; black 

 = dark blood. 



Thence it goes to the 



