VASCULAR SYSTEM OF THE FROG. 457 



The renal portal system, by which venous blood from the 

 posterior region passes 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. 

 Renal portal ) and receives the sciatic from the back of 

 system. J 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. 



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 trun- 

 cus 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. 



Lymphatic System. — The lymph is a colourless fluid, which 

 may be described as 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 posteriorly near the end of the 

 urostyle ; the other two lie between the transverse processes 

 of the third and fourth vertebrse. 



We must now return to the heart to consider how it is that 



