VASCULAR SYSTEM 



301 



a differentiation of the muscular trabeculse of the walls of 

 the heart. The atrium, into which the blood enters, represents 

 primitively the venous portion of the heart ; the ventricle, from 

 which the blood flows out, corresponding to the arterial portion. 

 The venous end further becomes differ- 

 entiated to form another chamber, the 

 sinus venosus, and the arterial end gives 

 rise distally to a truncus arteriosus ; the 

 proximal end of this (conns arteriosus) is 

 provided with more or less numerous 

 valves, and its distal end (bulbus arteriosus} 

 is continued forwards into the arterial 

 vessel (ventral aorta). 



The ventral aorta gives off right and 

 left a series of symmetrical afferent bran- 

 chial arteries (Figs. 242, 243), each of 

 which runs between two consecutive gill- 

 clefts, branches out into capillaries in the 

 gills, when present, and then becomes con- 

 tinuous with a corresponding efferent 

 branchial artery. After the first pair of 

 these has given off branches to the head 

 (carotids), they all unite above the clefts 

 to form a longitudinal trunk on either 

 side, and there form the right and left roots 

 of the dorsal aorta ; this extends back- 

 wards along the ventral side of the ver- 

 tebral axis into the tail as a large unpaired trunk, which gives off 

 numerous branches including paired vitelline or omphalo-mesenteric 

 arteries to the yolk-sac, and (except in Fishes and Dipnoans) 

 allantoic arteries to the embryonic urinary bladder or allantois 

 (pp. 9 and 337, and Figs. 8, 9, 242, 244). 



Primarily, the blood becomes purified in the vessels which 

 branch out over the yolk-sac, from whence it is returned by the 

 vitelline or omphalo-mesenteric veins (Fig. 244). These join with 

 the allantoic veins and veins of the alimentary canal to form what 

 eventually becomes the hepatic portal vein, which divides up into 

 capillaries in the liver. These capillaries then unite to form the 

 hepatic veins, which open into the sinus venosus. 



Into the sinus venosus there also opens on either side a pre- 

 caval vein or anterior vena cava, which receives an anterior cardinal 

 or jugular vein from the head, and a posterior cardinal vein from 

 the body generally (not including the alimentary canal). The 

 caudal vein which lies directly below the caudal aorta, is con- 

 nected with the posterior cardinals, usually indirectly, through the 

 renal portal veins (comp. Fig. 264). The further development of 

 the embryonic vessels may take place in one of three ways. 



The embryo may either leave the egg, and take on an aquatic 



A 



Sw 



FIG. 241. DIAGRAM SHOW- 

 ING THE PRIMITIVE RE- 

 LATIONS OF THE DIFFER- 

 ENT CHAMBERS OF THE 

 HEART. 



Sv, sinus venosus, into 

 which the veins from the 

 body open ; A, atrium ; 

 V, ventricle ; Ca, conus 

 arteriosus ; Ba, bulbus 

 arteriosus, from which the 

 main artery arises. 



