VI 



MOKPHOLOGY OF THE HEART 



371 



R 



B 



With increasing growth in length of the cardiac tu!>r this 

 curvature becomes converted into a double tloxure the heart taking 

 on a S-shape. Of the two curves which make up the 5 one which 

 has its concave side towards 

 the head represents the orig- 

 inal loop, while the other 

 which is convex towards the 

 head has developed in the 

 portion of cardiac tube lying 

 posterior to the primary loop. 

 Of these two curves the one 

 last mentioned, that which is 

 morphologically posterior, is 

 in an approximately verti- 

 cal plane. The anterior or 

 primary curve on the other 

 hand shows much variation 

 in position in different 

 Vertebrates. While on the 

 whole it still bulges towards 

 the right side, as did the 

 primary loop, the portion of 

 it formed by the originally 

 headward section of the tube 

 comes in many cases to lie 

 ventral to the other limb of 

 the curve. In other cases 

 this, originally anterior, por- 

 tion of the tube lies for a 

 time dorsal to the other, as 

 is the case in Salamandra. 

 The difference will be appre- 

 ciated by comparing the 

 relative positions of c and V 

 in Figs. 184, A, and 178. 



Of all the lower verte- 

 brates in which the peri- 

 cardiac space is still bounded 

 by rigid inextensible walls 

 it is the group of Lung- fishes 

 that shows the heart at 

 the highest level of evolu- 

 tion. And in correlation 

 with this fact we find that 

 in these fishes the kinking 

 of the cardiac tube attains 

 Lepidosiren (see below, pp. 

 cardiac tube (the "conns 



Fi. 177. A, diagram to illustrate the flexure of 

 the cardiac tube in the adult Lepidosiren, as seen 

 from tlie ventral side. The portion of the diagram 

 above the horizontal line represents the conus : 

 the portion below the horizontal line would 

 ivpivsent the rest of the heart on the assumption 

 that this portion of the cardiac tube ; 

 similar curvature to that of the conus. Longi- 

 tudinal lines drawn along the tube mark the 

 originally dorsal (D), ventral (I"), right (R), and 

 left (L). 



H shows the spiral twisting produced by straightening 

 out a tube possessing the sam flexure as the couus 

 portion in diagram A. 



its maximum. In a fully developed 

 376-378) the anterior portion of the 

 arteriosus ") has developed a further 



