56 COMPARATIVE ANATOMY 



mingle with the blood brought to those organs by the renal 

 and hepatic arteries respectively. Similarly, the blood 

 returned from the stomach and intestines has to pass a 

 second time through capillaries in the liver. Eventually, 

 the blood from the capillaries of the kidneys and liver finds 

 its way by the inferior vena cava into the sinus venosus. 

 When veins do not pass direct to the heart from the organs 

 and tissues in which they originate, but go to some other 

 organ and sub-divide in it to join its capillary system, they are 

 called portal veins, and the whole system of double capillary 

 circulation is called a portal system. Thus in the frog we 

 have a renal-portal and a hepatic-portal system, and renal- 

 portal and hepatic-portal veins. These portal systems are 

 very characteristic of the great group of Vertebrate animals, 

 but a renal-portal system is only found in its lower members. 

 The higher Vertebrata have only a hepatic-portal system. 



The walls of the sinus venosus are feebly muscular and 

 contractile. The beat of the frog's heart starts in the sinus 

 venosus, which, by its contraction, forces blood through the 

 sinu-auricular aperture into the right auricle, whence its return 

 is prevented by the valves by which that aperture is guarded. 

 At the same time the left auricle is filled with blood returned 

 from the lungs by the pulmonary veins. The blood brought 

 into the sinus by the venae cavas, and passed thence into the 

 right auricle, has passed through the capillaries of the body, 

 where it has been robbed of a considerable portion of its 

 oxygen, and has received a quantity of carbonic acid. In 

 consequence of its poverty in oxygen it has a dark purplish 

 hue, and is known as venous blood. But that coming from 

 the lungs has parted with its excess of carbonic acid, and has 

 taken up oxygen from the air contained in the lungs ; it is of 

 a bright scarlet colour, and is known as arterial blood. 



The muscular contraction which started in the sinus 

 venosus spreads over the auricles, which contract simul- 

 taneously, and drive the blood forward through the wide 

 auriculo-ventricular aperture into the single ventricle. The 

 return of blood from ventricle to auricles is prevented by the 

 membranous valve described above. As the right auricle 

 contains venous and the left auricle arterial blood, the 

 transversely-elongated cavity of the ventricle is filled with both 

 kinds, the venous blood being on the right side nearest the 



