. Text-book of embryology. Embryology. VI MOEPHOLOGY OF THE HEAET 371. V D R With increasing growth in length of the cardiac tube this simple curvature becomes converted into a double flexure the heart taking on a S-shape. Of the two curves which make up the S one which has its concave side towards the head represents the orig- D 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.
. Text-book of embryology. Embryology. VI MOEPHOLOGY OF THE HEAET 371. V D R With increasing growth in length of the cardiac tube this simple curvature becomes converted into a double flexure the heart taking on a S-shape. Of the two curves which make up the S one which has its concave side towards the head represents the orig- D 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 F'G- 177.—A, diagram to illustrate the flexure of • "U, -i CM A anr) 1 78 ^e cara^ac tube in the adult Lepidosiren, as seen 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 its maximum. In a fully developed Lepidosiren (see below, pp. 376-378) the anterior portion of the cardiac tube (the " conus arteriosus") has developed a further from the ventral side. The portion of the diagram above the horizontal line represents the conus: the portion below the horizontal line would represent the rest of the heart on the assumption that this portion of the cardiac tube possesses a s
Size: 1050px × 2380px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1910, bookpubli, booksubjectembryology
