54 



General Biology 



aerated forms the second type, or pulmonary circulation. The digested 

 food which the individual has absorbed must now be taken into the 

 blood and be made a part of that blood, so that there is a replacement 

 of lost substances. This explains why the blood, which goes to the 

 digestive tract by the coeliac axis, passes through two series of capillaries 

 before returning to the heart : 



ett 



cp.a 



Fig. 9. Diagram Representing the General Course of Blood in the Frog and the 

 Principal Sets of Capillaries (cp) Through Which the Blood Flows. 



The vessels through which impure blood goes are dark, while those carrying 

 pure blood are left unshaded. The arrows indicate the direction of blood flow. 

 ant. ab, anterior abdominal vein; ao' , aorta; au' , right auricle; au", left auricle; 

 c.c, common carotid artery ;cce, cceliaco-mesenteric artery; cp.a, anterior systemic 

 center; cp.al, alimentary center; cp.cu, cutaneous center; cp.hp, hepatic center; 

 cp.p, posterior systemic center; cp.pl, pulmonary center; cp.re, renal center; cu, 

 great cutaneous artery; d.ao, dorsal-aorta; l.h' ', anterior lymph heart l.h" ', posterior 

 lymph heart; in.cu, musculocutaneous vein; p. hepatic portal vein; p.cu, pulmo- 

 cutaneous vein; pr.c, precaval vein; pt.c, postcaval ve'm;pul, pulmonary vein; 

 re, renal artery; rp, renal portal vein; s.v., sinus venosus; tr.a, truncus arteriosus; 

 v, ventricle. (After Howes.) 



First, into the capillaries of the intestine where it receives the 

 nutriment absorbed from the food. Then, after being collected into the 

 large portal vein, it enters the liver. 



Second, after entering the liver, the portal vein breaks up into 

 another system of capillaries within that organ. 



After the blood has passed through the liver, this second set of 

 capillaries unites to form the hepatic vein which empties into the large 

 posterior vena cava leading to the sinus venosus. This whole system, 

 where veins break up into capillaries but are again united to form a 

 second vein, is called a portal system. 



Part of the blood which goes to the legs also has a double system. 

 First it enters the capillaries in the leg muscles. Then on its way back 

 it passes through the kidneys where it is broken up into capillaries. 

 The blood which takes this route returns from the leg through the renal- 

 portal vein, while the rest of the blood from the legs is diverted to the 

 abdominal vein which passes through the liver (but not the kidneys) 

 on its way to the heart. 



Now, just as the heart must have an arterial supply of blood to 



