CIRCULATORY SYSTEM 



783 



they are continuous with the large vitelline veins which bring the blood 

 back to the heart from the vascular area. 



The heart is now a short straight tube attached to the ventral wall 

 of the pharynx and consists of the muscular, united part of this "Y," 

 the two arms being the ends of the diverging vitelline veins which run 

 backward or caudad at the hindermost angle of the head fold. As this 

 fold is pushed farther and farther back, the straight part of the "Y" is 

 naturally pushed back also and lengthened. Not only this, but this 

 straight part of the heart grows more rapidly than does the place to 

 which it is attached, so that it does not even find room enough to con- 

 tinue its growth with the heartfold, but must bend into a loop with its 

 convexity toward the right side of the embryo. The heart has now lost 

 its attachment to the pharynx (with exception of its two ends). The 



Fig. 446. 



I, Schematic longitudinal sections of the heart. A, dogfish, B, Ganoids, and C, 

 Teolosts. a, atrium; 6, bulbus arteriosus (an enlarged portion of the truncus 

 arteriosus) ; c, conus arteriosus; k, valves; s, sinus venosus ; t, truncus arteriosus; 

 v, ventricle. (After Boas.) 



//, The circulatory system in the amphibians. A, Urodele, and B, Anura. 

 a.l. and a.r., left and right atria; ao.w., aortic root (radice) ; ca., carotid arteries, 

 which spring from the conus arteriosus together with the ao.w.; l.a., pulmonary 

 arteries which carry venous blood from the ventricle to the lungs ; l.v., pulmonary 

 veins which carry arterial blood to the left atrium ; v, the veins which carry venous 

 blood from the general body system to the right atrium ; ventr, ventricle. (From 

 Schimkewitsch after Wiedersheim.) 



caudal end of the heart, in which the vitelline veins empty, is called the 

 venous; the cephalic end the arterial end of the heart. Beating of the 

 heart begins as soon as a connection has been made between this "Y" 

 shaped tubular vessel with the vessels which have been formed in the 

 vascular area. The palpitation starts at the venous or caudal end and 

 passes to the arterial or cephalic end. The palpitation of the heart already 

 begins before one can distinguish any definite muscular tissue which has 

 developed from the mesoblast. 



The arterial end of the heart is known as the bulbus arteriosus. 

 From this, two narrow vessels, the aortic arches, pass around the diges- 

 tive tract to the dorsal side, turning caudad and becoming the dorsal 

 aortae. These two dorsal aortae run along each side of the notocord 

 under ;he mesoblastic somites and pass toward the tail unconnected 



