54 



GENERAL BIOLOGY 



be able to pump the blood to all parts of the body after it has entered 

 the heart from the lungs where it was aerated. The blood sent to be 

 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 : 



IT-.O. 



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 ; err, coeliaco-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; I h", posterior 

 lymph heart; m.cu, musculo-cutaneous vein; p, hepatic portal -vein; pcu, pulmo- 

 cutaneous vein; pr.c, precaval vein; pt.c, postcaval vein; 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 w r here it. receives the nutri- 

 ment absorbed from the food ; and, after being collected into the large 

 portal vein, enters the liver. 



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

 other system of capillaries w r ithin that organ. v j * 



After the blood has passed through the liver this second set of capil- 

 laries unites to form the hepatic vein which empties into the large pos- 

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

 vein's break up into capillaries but are again united to form a second vein, 

 is called a portal system. 



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

 first entering the capillaries in the leg muscles and on its way back pass- 

 ing through the kidneys where it is; broken up into capillaries: The 

 bl6od that takes this route returns from the leg through the renal-portal 

 vein, but 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. : 



