262 THE MECHANICS OF THE HEART 



cava which in all probabiUty is fully loaded with the waste products 

 of the fetal tissues. 



On account of the peculiar position of the orifice of the inferior 

 cava and the presence of a lip-like membrane, known as the Eustachian 

 valve, the blood entering the right auricle is immediately directed 

 through an opening in the interauricular septum into the cavity of the 

 left auricle. This orifice which thus grants a free passage to a portion 

 of the venous blood into the arterial side of the heart, is called the 

 foramen ovale. Under normal conditions this defect is closed very 

 shortly after birth, its place being taken by a tense fibrous membrane 

 which forever thereafter remains sharply differentiated from the 

 much thicker muscular portion of this septum. In certain infants, 

 however, it does not become patent until several weeks after birth; 

 in fact, in some it never becomes completely impervious. The venous 

 blood then continues to intermingle with the arterial and the more so, 

 the larger the size of the opening remaining. In indication of the 

 poor aeration of the tissues resulting in consequence of this condition, 

 the skin and mucous membranes of these children retain a bluish 

 appearance. 



From the left auricle, the blood passes into the left ventricle and 

 from here into the aorta. If it is now diverted into the blood-vessels 

 of the head, it eventually reaches the right auricle by way of the supe- 

 rior vena cava. Peculiarly enough, the stream from this blood-vessel 

 is directed in such a way that it flows directly through the right 

 auriculoventricular opening into the ventricle of the same side without 

 seriously interfering with the cross-current through the foramen ovale. 

 The pulmonary artery then conducts the blood into the lungs, but as 

 these organs are inactive and are merely a slowly growing mass of 

 tissue, they do not require much blood. For this reason, by far the 

 largest quantity of the blood of the pulmonary artery is not distributed 

 to the lungs at all, but escapes into the aorta by way of a special chan- 

 nel, commonly called the ductus arteriosus. Only an insignificant 

 portion of the blood of the pulmonary artery actually reaches the capil- 

 laries of the lungs, whence it again attains the left auricle by way of 

 the pulmonary veins. This blood, of course, serves solely the purpose 

 of supplying nutritive material to the growing lung tissue. 



A droplet of blood may pursue the course just outlined a number of 

 times, but it may also happen that it is forced into the posterior parts 

 of the body, i.e.., into the portal circuit or into the blood-vessels of the 

 legs, and eventually regain the heart by way of the inferior cava. Last 

 of all, it may leave the fetus altogether and return to the placenta by 

 way of the hypogastric branches and the umbilical artery. Clearly, 

 therefore, the paths which a drop of blood may follow, are even more 

 numerous and diverse in the fetus than they are in the adult. It may 

 be said, however, that a very considerable portion of the blood allotted 

 to the posterior part of the body, constantly leaves the fetal channels 

 to be replenished in the placenta. Considered in a general way, it is 



