460 



guise their own characteristic rythm. Thus the rush of blood caused 

 by the contractions of the zoöids is so strong that the ampullae cannot 

 withstand it, even during the most vigorous part of their systole. The 

 effect of the opposed heart beats is only exceptionally strong enough 

 to prevent the emptying of the ampullae but it can often be seen to 

 be strong enough to delay the systole for 5 seconds. Accordingly its 

 effect on the coordination must be pronounced, for any ampulla that 

 might otherwise have contracted a little ahead of the others will be re- 

 tarded, and the rest can catch up, so that all will empty their contents 

 together. The blood pressure theory is in perfect harmony with the 

 fact already mentioned that the ampullar contractions are less rigidly 

 coordinated in those parts of the vascular system that are considerably 

 removed from the zoöids and connected with them by few vessels; for 

 here the variations of pressure are less, as they depend mostly upon 

 the weaker contractions of the ampullae. 



The behavior of isolated ampullae is also explained ; for, given 

 their inherent tendency to execute rythmical contractions, all will tend 

 to contract together, as before the isolation. But, this being impossible, 

 the stronger ones will force the blood into the weaker ones whose 

 systole will be delayed until the diastole of the stronger ampullae. The 

 rare cases of adjacent ampullae executing alternate contractions may 

 be explained in the same way. For here, though the ampullae are not 

 isolated, the differences of pressure coming from a distance are small, 

 so that it might happen occasionally that an exceptionally weak am- 

 pulla would be influenced more by a neighboring strong one than by 

 the general pressure caused by more distant ampullae. In this case it 

 would contract alternately with the adjacent rather than with the di- 

 stant ampullae. 



In one respect, however, this theory encounters a difficulty. This 

 is in the case of isolated portions of the vascular system which become 

 functionally divided into several regions, the ampullae of each region 

 contracting together and forcing the blood into other regions. The 

 apparent »purpose« of the apparatus is to produce the most vigorous 

 circulation possible , and it is accomplished with wonderful success. 

 But if the path of least resistance were followed adjacent ampullae 

 would contract alternately instead of adjacent regions ; for it Avould be 

 easier for the contracting ampullae to force the blood to the next am- 

 pulla than to another region. 



There are two ways to explain this discrepancy. It may be said 

 that it is only the weakest ampullae that have the tendency to yield to 

 all pressures, that normally a slight pressure acts as a stimulus to con- 

 traction, and that therefore the ampullae do not seek the path of least 



