VASCULAR SYSTEM 



301 



a differentiation of the muscular trabecule 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 renosus, and the arterial end gives 

 rise distally to a truncus arteriosus; the 

 proximal end of this {conus arteriosus) is 

 provided with more or less numerous 

 valves, and its distal end (bulhus arteriosus) 

 is continued forwards into the arterial 

 vessel {ventral aorta). 



The ventral aorta gives off right and 

 left a series of symmetrical afferent iran- 

 ehial 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 arfe'ry. 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 /lepafe^jortaZ wm, which divides up into 

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

 hepatic reins, which open into the sinus venosus. 



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

 coval vein or anterior vena ivwo., which receives an anterior cardinal 

 01 jugular rein 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 



-Bev 

 -Cec 



—A 



—Sv- 



Fig. 241. — Uiaokam show- 

 ing THE PkIMITIVE RE- 

 LATIONS OF THE DlFfER- 



EXT Chambers of the 

 Heart. 



Sv, sinus venosus, into 

 ■sv-hich the veins from the 

 body open; ^-1, atrium; 

 V, ventricle ; Cos, conus 

 arteriosus ; Ba, bulbus 

 arteriosus, from which the 

 main artery arises. 



