196 MANUAL OF ANIMAL BIOLOGY 



passes by way of the renal portal vein into the kidneys. In Mam- 

 mals the renal portal system is lacking. (W. f. 121.) 



The hepatic portal system consists of a single large vessel, the 

 hepatic portal vein, which receives the venous blood with ab- 

 sorbed foodstuff from the alimentary tract by means of the in- 

 testinal, duodenal, and gastric veins. This blood is then 

 passed into the liver. After passing through the liver it is col- 

 lected by the hepatic vein, thence into the posterior vena cava. In 

 addition to the blood received from the alimentary tract through 

 the portal vein, the liver also receives a considerable supply of 

 venous blood directly from the hind limbs, body wall, etc., 

 through the ardominal vein. (W. fs. 115, A; 121.) 



Lymphatic System. The importance of the lymphatic system 

 has been noted above. In the Frog, it consists of a connected 

 series of lymph spaces and channels which permeate all the tissues 

 of the body. These range in size from microscopic spaces around 

 the cells to the very large lymph sacs which lie just under the skin 

 in various parts of the body. Connected with certain of the 

 larger lymph sacs are four special areas of contractile muscle tissue 

 which constitute the lymph hearts. They pump the lymph back 

 into the regular blood stream through certain veins. (W. fs. 124, 

 125.) 



The lymphatic system in the higher Vertebrates is characterized 

 by the development of numerous thin-walled lymph vessels which 

 connect with the lymph spaces. The lymph vessels empty into 

 the large thoracic duct which opens into the left jugular near the 

 heart. There are no definite contractile elements in this system, 

 and the flow of lymph through the thoracic duct and into the 

 regular blood channels is irregular and more or less dependent 

 upon the character of the body movements. (W. f. 115.) 



Course of Circulation in the Frog. The general course of 

 the circulation in the Frog is as follows ( W. 121, B) : 



The venous blood from all parts of the body, with the excep- 

 tion of the lungs, flows into the sinus venosus. The latter is 

 contractile, and it forces the blood into the right auricle. When 

 the right auricle contracts the blood is forced through the auriculo- 

 ventricular aperture and into the right side of the ventricle. By 

 the almost synchronous contraction of the left auricle, the newly 

 oxygenated, or arterial, blood, which has been received from the 

 lungs through the pulmonary veins, is also forced through the 



