HEART 



249 



from the air-bladder, passes right through the sinus venosus to 

 the left side of the atrium, and discharges the arterial blood 

 almost straight into the ventricle. This incipient division of 



Ai/fcZ 



a4< 



Fig. 218. 



Heart of Protopterus dnneetens, Owen. A, dorsal view. B, opened, ventral view. C, the 

 sinus venosus opened, a, cut arterial arches ; a 1 and 2, entrance from conns to first two 

 .arches ; a 3 and 4, entrance to last two arches ; at, atrium ; c, conns arteriosus ; c. w.c, cut wall 

 ofconus; c.w.v, cut wall of ventricle ; /, fibrous plug closing the passage from ventricle to 

 atrium and passing into the sinus ; I, dorsal attachment to pericardial wall ; l.<l, left ductus 

 Cuvieri ; Iv, longitudinal ridge ; p, small portion of pericardial wall ; p.r, pulmonary vein ; r.d, 

 -right ductus Cuvieri ; sv, longitudinal compound valve ; v, ventricle ; v.c, vena cava inferior ; 

 vv, row of small valves ; w, dotted line indicating course of venous blood from the shallow 

 sinus venosus, through the atrium (C) into the ventricle on the right of the plug (B). In C the 

 dotted lines Id, rd, and vo pass into the sinus venosus ; the lines as and y into the cavity of the 

 .atrium opening widely into the sinus ; the line p.v passes down the pulmonary vein to enter 

 the ventricle on the left of the plug (Z in B). 



the heart into a venous and an arterial channel is continued in the 

 atrium, where an incomplete interauricular wall is developed as a 

 thick projecting plug, and is even carried on into the ventricle. Of 

 the four main longitudinal rows of valves found in the spirally 



