vi CIRCULATION 93 



becomes filled with non-aerated, the left with aerated 

 blood, the two taking an appreciable time to mingle, 

 owing to the spongy character of the ventricular wall. 



The instant it is thus filled, the contraction of the 

 ventricle begins. As it does so . the blood, getting 

 behind the auriculo-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 enter- 

 ing 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 alter- 

 native courses are now open to the blood : it can pass 

 either directly from the conus into the pulmo-cutaneous 

 trunk (pul. cu. tr], or make its way into the bulbus 

 aortae (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 

 flowing onwards i.e., away from the heart in the 

 systemic and carotid trunks. Consequently the pressure 



