200 STUDIES IN ADVANCED PHYSIOLOGY. 



From these regions, that is from head and neck, the 

 blood is again gathered up by veins, and finally by the vena 

 cava descending, carried to the right auricle. But here 

 this current of venous blood is so poured into the right 

 auricle that it does not meet the current of pure blood flow- 

 ing through it to the left auricle. The descending vena 

 cava opens into the auricle at the opposite side and from 

 above, and in this way the stream of venous blood which it 

 bears at once drops through into the right ventricle. This 

 crossing of two streams, one arterial and one venous in the 

 right auricle, may be easily understood by showing how 

 readily one might construct pipes leading into a room in 

 such a way as to have two currents flow through it and yet 

 have them separate and distinct; having one, for instance, 

 in one corner drop into the room from above, into a collect- 

 ing funnel below, while the other stream might at an oppo- 

 site corner be carried across from one wall to another inde- 

 pendently of the former, especially if a flap or plank like 

 the Eustachian valve should be added. 



The stream of venous blood having reached the right 

 ventricle in the manner described, is now by the systole of 

 the right ventricle forced out into the pulmonary artery, 

 and would naturally go to the lungs ; but the lungs do not 

 at this time contain air. They are collapsed, and it would 

 be almost impossible and entirely useless to force this 

 stream of blood through them. This difficulty is remedied, 

 however, by a communicating branch which connects the 

 pulmonary artery with the arch of the aorta, and so enables 

 the venous blood to pass from the pulmonary artery through 

 this connecting duct into the descending aorta. This con- 

 necting duct is called the duct of Botallus. As this duct 

 of Botallus reaches the aorta after it has given off its vessels 

 to the head it prevents the flow of this venous stream in 

 that direction. But this venous stream, together with a 

 little pure blood which finds its way through the arch of the 

 aorta from the left ventricle, descends ^ through the aorta, 

 and while a little of it is carried by arteries which supply 



