198 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



jugulars of the two sides. In the mammals (Fig. 208) a trans- 

 verse connection forms between the precavae of the two sides ; 

 and then the direct connection of the left precava with the heart 

 is lost, so that all the blood from the right side of the head and 



FIG. 208. Diagram (altered from Gegenbaur) showing the modifications of 

 the venous trunks in mammals, tf , azygos vein ; c t coronary ; d, ductus Cuvierii ; 

 ei t external iliac; ej, external jugular; //, hepatic; ha, hemiazygos ; ic, intercostals; 

 zY, internal iliacs; //, internal jugulars; //, left innominate; Ic, left posterior cardi- 

 nal; /, precava; po, postcava; r, renal; ;r, right posterior cardinal; ri, right 

 innominate; s, sinus; sc, subclavian ; si, superior intercostal. In B the postcava 

 has extended backwards and tapped the right posterior cardinal ; and a transverse 

 trunk has formed between the jugulars of the two sides. In Ca transverse vessel, 

 /, has united the two postcardinals ; and these have lost their other connections, and 

 form the azygos system. 



the right fore limb passes through the left precava in its way to 

 the heart. 



The lymph system forms another series of circulatory vessels 

 which are distinct from the blood-vessels, excepting at one or 

 more points where they connect, the lymph flowing from the 

 lymph vessel^ into the venous system. The walls of the lymph 

 vessels are always thin ; in most places they consist merely of 



