CIRCULATORY SYSTEM 793 



lower tetrapoda where the femoral artery is small. It is likewise char- 

 acteristic of the embryos of mammals. In the latter, however, b.efore 

 birth, the femoral artery grows down to join the popliteal, and so be- 

 comes the chief supply of the limb. These trunks and the hypogastric 

 do not always remain distinct. They often fuse in different ways at the 

 base. Epigastric and hypogastric arteries are distinct in many reptiles 

 and in birds, but in other vertebrates they fuse to form the common iliac 

 artery, so called because the proximal portion of the femoral is often 

 called the external, and the hypogastric the internal iliac artery. The 

 sciatic, likewise, may remain distinct, or it may fuse with the other3 at 

 the base, and then its independent portion will appear as a branch of the 

 common iliac artery. 



A cutaneous artery arises from either the subclavian or the pul- 

 monary artery of either side (both conditions occur in the amphibia) to 

 run backward in the skin of the trunk. It may extend back and unite 

 with the epigastric artery. If, as in the amphibia, these arise from the 

 pulmonary artery, they contain venous blood and the skin acts as a sub- 

 sidiary respiratory organ. 



Renal Arteries: 



Renal arteries are paired and show metamerism in the primitive 

 state, details of which are given in the description of the organs they 

 supply. It is well to note that metamerism is well shown in these arteries 

 going to the pronephros and the mesonephros, while in the true kidney 

 the metanephros only a single pair of renal arteries furnishes the blood 

 supply. 



Genital Arteries: 



These, like the renal arteries, are paired and metameric in the primi- 

 tive state and are called 

 Spermatic in the male. 

 Ovarian in the female. 

 These are more numerous in lower animal forms than in higher. 



THE VEINS 

 Omphalomesenteric Veins : 



It will be remembered that the heart is developed in the pericardial 

 cavity. Caudad to the heart region the liver begins developing and tHus 

 prevents the lateral plates from coming together on the ventral side 

 as they did in the case of the heart. The lateral plates, however, become 

 grooved, and each one forms a tube, so that there are two vessels, called 

 the omphalomesenteric veins extending caudad from the heart, passing 

 around the liver where they meet with the extensions of the omphalo- 

 mesenteric arteries already described (Fig. 277). 



