CIRCULATION 



93 



ventricular valves, forces them together, and thus prevents any 

 backward flow into the auricle. At the same time the 

 semilunar valves at the entrance of the conus {c art) are 

 pushed aside and the blood flows into that chamber. Since 

 the conus opens from the right side of the ventricle, the blood 

 first entering it will be non-aerated ; there will then follow a 

 certain amount of mixed blood ; and finally, as the ventricle 

 reaches the limit of its contraction, the aerated blood 

 from its left side will be forced into the conus. (Compare 



Fig. 22.) 



Last of all the chambers of the heart, the conus begins 

 its contraction. The semilunar valves are immediately 

 filled with blood, and, closing together, stop all backward 

 flow into the ventricle. Two alternative courses are now 

 open to the blood . it can pass either directly from the 

 conus into the pulmo-cutaneous trunk {pul. ai. /r), or make 

 its way into the bulbus aortte {b. ao). As a matter of fact it 

 takes the former course, owing to the circumstance that 

 there is little resistance in the limited blood-system of the 

 lungs, while that in the systemic and carotid trunks is very 

 great. Hence the blood just received into the conus from 

 the ventricle, which, as we have seen, is non-aerated, goes 

 immediately to the lungs and skin to be aerated. 



Before long — in a fraction of a second — the flow of 

 blood into them increases the pressure in the pulmonary 

 vessels, and at the same time the blood is continually flow- 

 ing onwards — i.e., away from the heart — in the systemic and 

 carotid trunks. Consequently the pressure in these vessels 

 .rapidly diminishes, and the blood soon forces aside the 

 valves between the conus and the bulbus and fills the latter. 

 Here again the question of pressure comes in. It is easier 

 for the blood to make its way into the wide systemic trunks 

 {syst. tr) uniting immediately into the long dorsal aorta 



