56 COMPARATIVE ANATOMY 



the trunk and the abdominal viscera takes a much more 

 complicated course. 



The blood brought from the hind limbs by the femoral 

 vein may take one of two paths. It may either go by way 

 of the iliac veins to join the blood brought by the sciatic 

 veins, and pass on by the renal-portal veins to the kidneys; 

 or it may go by way of the pelvic and anterior abdominal 

 veins to the liver. In either case it has, whether in the 

 substance of the kidney or in the substance of the liver, to 

 pass a second time through a system of capillaries, and to 

 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 Verte- 

 brate 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 cavae, 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 



