ORGANS OF CIRCULATION. 



183 



aortic arch of either side arise two arteries, the external (ven- 

 tral) and internal (dorsal) carotids, which run forward to supply 

 the head ; the external being distributed to the muscles of the 

 head and tongue, the internal going up through the floor of the 

 skull to the brain. 



Farther back the dorsal aorta gives off vessels, right and 

 left, to the adjacent seg.ments (see p. 188), and then gives off 

 two larger trunks, the omphalomesaraic arteries, which at first 

 connect directly with the omphalomesaraic veins already men- 

 tioned. Behind the origin of these arteries the dorsal aorta is 

 a paired structure, but soon the two halves unite into the single 

 vessel found in the adult of all forms. Near the posterior end 



FlG. 197. Diagram of early circulation in a vertebrate with small yolk. DC, 

 ductus Cuvierii; EC, external carotid; H, heart; HA, hypogastric artery; 1C, 

 internal carotid; J, jugular vein; L, liver; M, point of formation of mouth; 

 OA, omphalomesaraic artery ; O V, omphalomesaraic vein ; PC, posterior cardinal 

 vein ; V, vent. 



of the peritoneal cavity the dorsal aorta gives off a pair of hypo- 

 gastric arteries which run downward on the side of the alimen- 

 tary canal, and behind these the aorta continues into the tail as 

 the caudal artery. 



A little later another system of veins arises. These are the 

 jugulars, or anterior cardinals, and the posterior cardinals. These 

 run on either side of and a little below the backbone, the jugu- 

 lars coming from the head, the posterior cardinals from the dorsal 

 wall of the body cavity. These vessels of either side unite just 

 above the heart into a transverse vessel, the ductus Cuvierii, 

 which empties into the sinus venosus. 



The foregoing gives in outline the great vascular trunks of 

 the body ; but these undergo many modifications in the different 



