THE CIRCULATORY SYSTEM 



IN no other system of organs does the fundamental law of 

 biogenesis find such wide application as in the circulatory, and 

 to go into details concerning it would be merely to repeat 

 what has been often said before. Attention may therefore be 

 confined to the following facts. 



THE HEAKT 



The heart arises (cd., Fig. 31, A), at an early embryonic 

 stage, far forwards in the cervical and indeed in the cephalic 

 region. This recalls its position in adult Fishes and Amphibians. 

 The comparison with these animals is the more fully justified, 

 in that the heart of the early human embryo, like that of the 

 lowest Ariamnia, has throughout a single lumen, and its further 

 differentiation is gradually undergone in correspondence with the 

 phylogenetic development of the organ. 



The structure of the heart, originally "very simple, soon 

 becomes complicated, but even then certain peculiarities of the 

 right auricle point back to the condition found in the Amphibia. 

 These are, for example, the inconstant vestiges of valves at the 

 opening of the left vena cava superior (Thebesian valve), and 

 the almost constant remains of the valves of the sinus at that of 

 the vena cava inferior (Eustachian valve). The same applies to 

 the traces of the incorporation of the sinus venosus and of the 

 pulmonary veins into the opposite divisions of the atrium 

 (auricles). In short, Comparative Anatomy furnishes not only 

 interesting parallels with, but an explanation of the various stages 

 in the Ontogeny of the heart of the higher Vertebrata. There 

 are, however, some conditions which occur in the Mammalian 

 heart, especially during the early periods of its development, 

 which cannot be explained by inheritance, but which have arisen 

 secondarily through adaptation ; among the chief of these are the 



