28 LABORATORY STUDIES IN ZOOLOGY 



tliis branch is (he internal jtujidar, returning blood from tlic 

 interior of the skull, which it leaves by an aperture at the posterior 

 border of the orbit. 



c. The third and outer tributary of the precava is the subclavian 

 vein. It is the largest of the three and comes from the body wall 

 to the base of the arm where it receives the brachial vein from 

 that appendage. Before this point it is found as the large 

 miisculo-ciUaneous vein which returns blood from the skin and 

 muscles of the side and back of the body and from the head. 



The left precava in its course and tributaries corresponds to 

 the right precava. It may be located. 



In synoptic fashion (or otherwise), show: (a) the region drained 

 by the precava, (6) the principal tributaries of the precava and 

 the areas drained by each. 



The postcava is the only vein returning blood from the posterior 

 part of the body. It drains the kidneys and liver and through 

 them the remaining organs. Expose both kidneys ventrally by 

 removing the peritoneum and pushing back the other structures 

 which obscure them. Note how the postcava is formed between 

 the kidneys by the renal veins and follow it cephalad to the liver. 

 Dissect away enough of the substance on the dorsal side of the 

 left lobe of the liver to show that the postcava passes through this 

 organ without breaking up into capillaries. As it emerges from 

 the liver under the apex of the heart it receives the right and left 

 hepatic veins and passes directly into the sinus venosus. Harmo- 

 nize this dissection with the structural principle shown in Fig. 1 

 and relevant statements already made. 



Veins Entering the Left Auricle. — The pulmonary vein is formed 

 by the union of the right and left pulmonary veins and returns 

 blood to the heart from the lungs. Each pulmonary vein extends 

 along the inner side of the lung and may be located by dissecting 

 away, little by little, the membrane between the heart and the 

 lungs. 



The Portal Systems. — The liver is one of the organs in the 

 frog's body in which veins en route to the heart break up into 

 capillaries and then reunite to form veins before leaving the organ. 

 Because of this peculiarity of structure this portion of the venous 

 system is known as the hepatic portal system. There is another 

 portal system in the frog which is located in the kidneys and 

 called the renal portal system. The veins which carry blood to 



