LYMPHATIC SYSTEM. 595 



The renal portal system, by which venous blood from 

 the posterior region filters through the kidneys on its way 

 back to the heart, is as follows on each side : 



{A posterior branch of the femoral vein from the 

 hind-limb forms the renal portal vein, which 

 receives the sciatic from the back of the leg, and 

 the dorso-lumbar veins from the dorsal wall of 

 the body, and oviducal veins in the female. 



The anterior branch of the femoral vein is called the 

 pelvic, and unites with its fellow of the opposite side, and 

 gives origin to a median vein which runs to the liver the 

 anterior abdominal. By means of an anastomosing branch, 

 the anterior branch of the femoral is also connected to the 

 sciatic. 



The hepatic portal system, by which venous blood from 

 the posterior region and from the gut passes through the 

 liver on its way back to the heart, is as follows : 



(Anterior abdominal vein, from the union of the 

 two pelvics, receiving tributaries from the 

 bladder, ventral body wall, and truncus 

 arteriosus. 

 Hepatic portal vein, from the union of veins from 

 the stomach, intestine, and spleen. 



III. The pulmonary veins, which bring back purified 

 blood from the lungs, unite just before they enter the left 

 auricle. There are numerous valves in the veins of the 

 frog. 



Lymphatic system, The lymph is a colourless fluid, like blood 

 without red corpuscles. It is found in the spaces between the loose 

 skin and the subjacent muscles, in the pleuro-peritoneal cavity in which 

 heart, lungs, and other organs lie, in a sub-vertebral sinus extending 

 along the backbone, and in special lymphatic vessels which pass fatty 

 materials absorbed from the intestine into the venous system. There 

 are two pairs of contractile ' * lymph hearts " at two regions where the 

 lymphatic system communicates with the veins. A pair lie near the 

 posterior end of the urostyle ; the other two lie between the transverse 

 processes of the third and fourth vertebrae. Their pulsations can be 

 seen on the back of the living frog. 



Mechanism of the heart. The right half of the ventricle, 

 being nearer the right auricle, contains more impure blood, and it is 

 from the right side of the ventricle that the truncus arteriosus arises. 



The middle of the ventricular cavity contains mixed blood. The 

 left corner contains pure blood received from the pulmonary veins. 



