588 EMBRYOLOGY. 



during the last months. On account of theso facts, less blood 

 passes, even before birth, from the inferior vena, cava into the left 

 atrium, and likewise less from the pulmonary artery into the 

 descending aorta, than was the case in earlier months. Thus there 

 is gradually introduced toward the end of pregnancy a separation 

 into a right and a left heart, with their separate blood-currents 

 (HASSE). But it is almost at a single stroke that this separation, in 

 consequence of birth, becomes complete. 



Great alterations are now brought about by the beginning of 

 pulmonary respiration and by the cessation of the placental circulation. 

 Both events cooperate to increase the blood-pressure in the left 

 heart, and to diminish that in the right. The blood -pressure becomes 

 reduced because no more blood runs into the right atrium from the 

 umbilical vein and because the right ventricle must furnish more 

 blood to the expanding lungs. In consequence of this the ductus 

 Botalli (fig. 318 n) is closed and then converted into the ligamentuiu 

 Botalli. Since, moreover, a greater quantity of blood now flows 

 from the lungs into the left atrium, the pressure in the latter is 

 increased, and since at the same time the pressure is diminished in 

 the right atrium, the closure of the foramen ovale, owing to the 

 peculiar valvular arrangements, is now effected. For the margin of 

 the valvula foraminis ovalis applies itself firmly to the limbus 

 Vieussenii and fuses with it. 



By the closure of the oval foramen and the Botallian duct the 

 division of the blood -current into a major, systemic circuit and a 

 minor, pulmonary circuit, which was initiated before birth, is now 

 completed. 



SUMMARY. 

 Development of the Heart. 



1. In the first fundament of the heart two different types can be 

 distinguished in Vertebrates. 



First Type. In Gyclostomes, Selachians, Ganoids, and Amphibia 

 the heart is developed from the beginning as an unpaired 

 structure on the under [ventral] surface of the cavity of 

 the head-gut, in the ventral mesentery, which is thereby 

 divided into a mesocardium anterius and posterius. 



Second Type. In Birds and Mammals the heart is developed 

 out of separate halves, which afterwards fuse with each 

 other into a single tube, which then has the same position 

 as in the first type. 



