ANATOMY OF THE FROG 55 



courses forwards in the middle line of the ventral wall of the 

 abdomen till it reaches the xiphoid cartilage of the sternum. 

 It then turns inwards and divides into three branches, two 

 lateral and one median. The two lateral branches pass to the 

 right and left lobes of the liver respectively, and break up in 

 its substance into capillaries ; the median branch unites with 

 the hepatic portal vein. 



The last-named vessel brings blood from the stomach and 

 intestines. It is formed by the union of the intestinal, gastric 

 and splenic veins, runs forward in the mesentery, and passes 

 into the left lobe of the liver, where it breaks up into 

 capillaries. Just before it enters the liver the hepatic portal 

 vein is joined by the descending branch of the anterior 

 abdominal. The blood from the liver is collected by the right 

 and left hepatic veins, which open into the inferior vena cava 

 just before it passes into the sinus venosus. We have seen 

 that the lungs are supplied with blood by the two pulmonary 

 arteries, each of which courses down to the outer surface of 

 the lung sac to which it belongs from base to apex, giving off 

 a number of branches as it goes. The blood, after passing 

 through the lung capillaries, is gathered by venules, which unite 

 to form the venous vessel lying along the inner side of the 

 base of each lung. These right and left pulmonary veins run 

 above the corresponding vense cavae, and unite to form a 

 single trunk, the common pulmonary vein, which opens into 

 the left auricle. We are now in a position to review the 

 course of the circulation of the blood in the frog. The blood 

 from the head and fore limbs, as well as that brought back by 

 the musculo-cutaneous veins, passes straight into the sinus 

 venosus by way of the superior venae cavse. But the blood 

 coming from the hind limbs and from the posterior part of 

 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 



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