IQ2 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



The arteries already described extend ventrally from the 

 aorta; but there arise earlier numerous pairs of vessels, seg- 

 mentally arranged (see p. 188), which run out in a transverse 

 direction to the muscles and to the urogenital structures. 

 Those which run out in the body wall to supply the muscles, 

 etc., are known, according to their position, as the intercostal 

 and lumbar arteries ; those going to the excretory organs are the 

 renal arteries ; while the genital arteries (ovarian or spermatic) go 

 to the reproductive organs (gonads). In the lower vertebrates, 

 where the pro- or mesonephros is functional throughout life, the 

 renal arteries retain their metameric character ; but with the 

 formation of a metanephros (amniotes) the segmental arrange- 

 ment is lost, and the kidneys receive their blood through a single 

 pair of renal arteries. The blood-vessels supplying the gonads 

 undergo a similar reduction in the higher vertebrates. 



Veins. The primary venous trunks have been enumerated 

 on a previous page ; they are a pair each of omphalomesaraics, 

 jugulars, and posterior cardinals. 



The omphalomesaraic veins are the paired posterior continu- 

 ations of the grooves from which the heart is formed. They con- 

 tinue themselves backwards, and at first are connected with the 

 omphalomesaraic arteries (see Fig. 197). Soon this connection 

 is lost, and the vein of the right side partially disappears, while 

 the other sends out branches, right and left, over the yolk. In 

 those vertebrates which, like the sauropsida, have a large yolk, 

 these vitelline veins play an important part in the early develop- 

 ment, but with the absorption of the yolk they disappear. From 

 the point where the vitelline veins arise from the persistent om- 

 phalomesaraic, two veins grow back along the tail beneath the 

 caudal artery, passing on either side of the rectum. Fusion of 

 these vessels occurs, and there results a single caudal vein with 

 a loop around the vent (Fig. 205). 



In the beginning the two omphalomesaraics pass on either 

 side of the liver ; but as this organ develops, the left omphalo- 

 mesaraic sends a branch into it from behind, while from the 

 anterior side both omphalomesaraics extend into this gland. 

 There is thus inaugurated a system of circulation (the portal 

 system) through the liver, while that part of the left omphalo- 



