VEINS 307 



the peroneal supplying the calf of the leg, the others continuing into 

 the foot. 



The arrangement of vessels thus outlined is characteristic of the lower 

 tetrapoda where the femoral artery is small. It is also characteristic of the 

 embryos of the mammals, but in the latter, before birth, the femoral artery 

 grows down, joins the popliteal, and thus becomes the chief supply of the 

 limb. These trunks and the hypogastric do not always remain distinct, but 

 may fuse in different ways at the base. Epigastric and hypogastric arteries 

 are distinct in many reptiles and in birds, but elsewhere they fuse to form the 

 common iliac artery, so called since the proximal portion of the femoral is often 

 called the external, the hypogastric the internal iliac artery. The sciatic, too, 

 may remain distinct or it may fuse with the others at the base, and then its 

 independent portion appears as a branch of the common iliac artery. 



A cutaneous artery, arising from either the subclavian or the pulmonary 

 artery of either side (both conditions occur in the amphibia), runs backward in 

 the skin of the trunk, and may extend back and unite with the epigastric artery. 

 When, as in the amphibia, these arise from the pulmonary they contain venous 

 blood and the skin acts as a subsidiary respiratory organ (p. 277). 



The arteries going to the excretory and reproductive organs are 

 paired and, in the more primitive vertebrates show a marked meta- 

 merism. They are best described in details along with the urogenital 

 structures in a subsequent section. It may be mentioned here that 

 the metamerism is well shown in the nephridial or renal arteries 

 going to t he pro- an d mesonephroi, while there is usually but a single 

 pair of renal arteries to supply the metanephroi (true kidneys) of the 

 amniotes. /Thp arterips to the gonads may be included under the 



' single head ofgenital arteries, though they are usually subdivided 

 into the spermatic and ovarian arteries according to the sex. Like 



/ the nephridial, the genital arteries are more numerous in the lower 

 and are reduced in number in the higher forms. 



THE VEINS 



Continuing from the description of the development of the heart 

 (p. 294), the ventral margins of the lateral plates, posterior to the 

 pericardium are kept from meeting in the middle line by the anlage 

 of the liver (figs. 327, 328). The edges of the plates become grooved 

 in the same way as in front and each groove becomes rolled into a 

 tube, lined with vascular cells, so that two vessels, the omphalo- 

 mesenteric veins, extend back from the heart, around the liver, to 

 meet the extensions of the omphalomesenteric arteries already 

 described. Behind the connexion of these vessels another pair of 



