940 



THE ORGANS OF CIRCULATIOX 



month it is about 1 to 50; at l)irtli it is 1 to 120; whilst in the aduU it is about 

 1 to 160. 



Tlie auricular portion is at first more cai)acious than the ventricular, and the 

 right auricle is larger than the left. The ventricular walls are at first about 

 equal in thickness, l)ut at birtli the left wall is the thicker of the two. 



The foramen ovale is largest about the sixth month of intra-uterine life, and 

 there is \\\) to this })criod a direct communication between the two auricles. The 

 valve of the foramen ovale, however, gradually advancing upwards bej'ond the 

 annulus, on its left side, acts as a perfect valve during the latter half of foetal life, 

 and thus prevents the return of blood into the right auricle. 



The Eustachian valve has already been described; it is of large size in the foetal 



Fig. 558.— Anterior View of Heart and Great Vessels of Fcetus, the Anterior 

 Chest Wall beixc; removed axd the Heart Sac opened. 



Lffl Left 



caivlid si'Uluiiini SECTION OF CLAVICLE 



Duclus arleriosiis 

 Aoita 



Pulmonary artery 



PERICARDIUM 

 REFLECTED 



RIGHT AURICLE 



fV LEFT VENTRICLE LEFT AURICULAR 



CUT EDGE OF 

 PERICARDIUM 



UPFtR SURFACE RIGHT AURICULAH RIGHT 



OF DIAPHRAGM APPENDIX VENTkICLE 



lieart, and serves to direct the current of blood from the inferior cava into the left 

 auricle. 



The ductus arteriosus continues the i)ulmonarv artery onwards to the concavity 

 of the arch of the aorta, to a ])oint just beyond the origin of the left subclavian 

 artery, and so transmits the l)lood from the right ventricle into the descending 

 aorta. The ductus arteriosus during the growth of the ]nilmonarv artery becomes 

 ultimately associated with its left branch. 



The foetal circulation. — The right auricle of the heart receives its blood from 

 both vense cavie, as well as from the coronary sinus. That which is conveyed by 

 the superior cava is venous l)lood returned from the head and neck and upper 

 extremities. The inferior cava returns the blood from the lower half of the body, 

 as well as that which comes from the i)lac<'nta througli the umbilical vein. This 

 latter stream nniclies the inferior cava, in })art directly through the ductus venosus, 

 and in part through the liver and hepatic veins. 



