234 



TEXT-BOOK OF EMBRYOLOGY. 



as the individual continues to grow, the heart, generally speaking, increases in size 

 accordingly. This increase takes place by intussusception in the endocardium 

 and myocardium. At the time of birth the two atria are in communication 

 through the foramen ovale which is simply an orifice in the atrial septum (Fig. 208) . 

 Thus the blood which is brought to the right atrium by the body veins is al- 

 lowed to pass directly into the left atrium, thence to the left ventricle, and thence 

 is forced out to the body again through the aorta. A certain amount of blood 

 also passes from the right atrium into the right ventricle and thence into the 

 pulmonary artery; but this blood does not enter the lungs but passes directly 

 into the aorta through the ductus arteriosus (Fig. 207) . After birth the lungs begin 



Innominate artery' 



Branches of right a4^ 

 pulmonary artery" ~Z^z%&.. 



Arch of aorta 



Pulmonary artery 



Right auricular appendage- 



Right ventricle 



Left carotid artery 

 Left subclavian artery 



Ductus arteriosus 



|&?> s Branches of left 

 ~~y pulmonary artery 



,' 



Left auricular appendage 

 Left ventricle 



Descending aorta 



Fig. 207. — Ventral view of heart of foetus at term. Kallmann's Atlas. 



to function and the placental blood is cut off, so that the right atrium receives 

 venous blood only and the left arterial blood only. If the foramen ovale were to 

 persist it would allow a mingling of venous and arterial blood. Consequently 

 the foramen ovale closes soon after birth and the two currents of blood are com- 

 pletely separated. At the same time the ductus arteriosus atrophies and be- 

 comes the ligamentum arteriosum. Consequently there is no direct communica- 

 tion between the pulmonary artery and aorta. 



Certain features of development have an important bearing on the theories regarding the 

 physiology of the heart, particularly on the theory that the heart is an automatic organ. 

 Whether the theory that the heart beats automatically, i.e., independently of stimuli from 

 the nervous system, is true or not, it is a fact that in the embryo it begins to beat before any 



