THE COUESE OF THE CIECULATION. 419 



The right ventricle is thus filled with venous blood, and the 

 left ventricle with arterial blood. On the ventricular systole, 

 the arterial blood from the left ventricle is driven through the 

 aortic trunk and the carotid arteries to the head ; while the 

 venous blood from the right ventricle is driven through the 

 pulmonary trunk and pulmonary arches into the dorsal aorta, 

 and then backwards along the body, the greater part passing 

 along the large allantoic arteries to the placenta. 



The changes that occur at birth are practically the same as 

 those which are effected in the chick on hatching (c/. p. 314). 



(i) The vitelline and allantoic circulations are stopped. The 

 result of this is that the blood in the posterior vena cava is 

 from this time venous, since the arterial supply previously 

 brought by the allantoic veins is now cut off. 



(ii) The ductus venosus, or direct passage through the liver, 

 is closed. The effect of this change is that all the blood brought 

 to the liver must now pass through its capillaries in order to get 

 to the heart, whereas formerly the ductus venosus afforded a short 

 cut by which the liver capillaries could be avoided. 



(iii) The ductus arteriosus, or ductus Botalli, closes on both 

 sides of the body. This renders it impossible for blood from 

 the right ventricle to get directly into the aorta. All the blood 

 from the right ventricle has now to pass along the pulmonary 

 arteries to the lungs, and the pulmonary vessels consequently 

 dilate very considerably, to accommodate this increased quantity 

 of blood. A further effect is that the dorsal aorta now receives 

 its blood supply from the left ventricle instead of, as formerly, 

 from the right ventricle ; i.e. it now contains arterial, instead of 

 venous blood. 



(iv) The foramen ovale closes. This is efiected at a rather 

 later stage than the other changes. When it is completed, the 

 blood from all three venas cavee enters the right auricle, and is 

 driven from this into the right ventricle ; while the only blood 

 entering the left auricle is now the blood returned from the 

 lungs by the pulmonary veins, vessels which up to the time of 

 birth are comparatively small and insignificant, but which dilate 

 very greatly as soon as lung breathing is established. 



The circulation, by these changes, becomes that of the adult. 

 A complete double circulation is established ; the right and left 



E B 2 



