68 



THE MECHANISM OF THE CIRCULATION. 



is fourfold as great as in the first case. 1 The efficacy of an impulsive 

 compared with a steady force is a matter of everyday observation, and 

 we may perhaps attribute this result to the opening of the capillaries by 

 the impulsive action of wave motion. 2 



A DISCUSSION OF SOME OF THE GENERAL PRINCIPLES OF THE 



CIRCULATION. 



By E. H. Weber 3 the model represented in Fig. 45 was constructed. This 

 can be conveniently used as the basis for the discussion of some of the main 

 principles of the circulation. In this model, a piece of small intestine, 

 fashioned as a single chamber, represents the heart (H). The valves are 

 formed of short pieces of intestine, which are firmly tied on to pieces of glass 



tube. These are 

 fixed within and to 

 either end of the 

 heart. The valves 

 are prevented from 

 turning inside out 

 under pressure 

 by short guiding 

 strings, which, 

 fastened as they 

 are to the edges of 

 the valves, take the 

 place of the chords 

 tendineae. The 

 artery (A) and the 

 vein (V) are formed 

 FIG. 45. of a long loop of 



intestine, and a 



sponge (0) is inserted within this loop to take the place of the arterioles and 

 capillaries. The model is distended with water to an extent represented by 

 m and n in the manometers D and D'. 



Suppose the model be laid upon a horizontal surface, so that the effect of 

 gravity is eliminated, and the conditions made as simple as possible. If by 

 squeezing it with the hand, H be now made to contract, the fluid contained 

 therein is directed by the valves and forced on into AA. If AA were a 

 rigid tube, the whole column of fluid would be forced on at one and the same 

 time, and the flow of fluid into V would last only just so long as the contrac- 

 tion of H was maintained. As, however, AA is an extensile elastic tube, 

 the projection of the fluid is successive. The systolic output first finds a 

 place by distending the tube (A), and produces therein a positive wave, which 

 travels with a certain velocity through the system. This velocity is inde- 

 pendent of the velocity of flow. If the valves were not competent, and the 

 systole passed into diastole at the moment the blood was expelled, then the 

 expanded tube (A) would drive part of the blood back again into H, and a 

 negative wave would arise, which, following the positive wave, would also travel 

 with a certain velocity through the system. In the entire absence of valves, the 

 fluid particles would swing backwards as much as forwards, and in the end 

 would suffer no change of place. As, however, the valves are competent, there 

 is no back swing into H, and each systole pushes on the fluid a step further. 



If all valves were absent from H, the fluid would stream into it from 

 both sides of the system at the moment when the pressure on it was relaxed ; 



1 Hamel, Ztsclir. f. BioL, Munchen, 1888, Bd. xxv. S. 474. 



2 Nicolls, privately communicated to the author. 



3 Bcr. u. d. Verhandl. der ktinigl. Sachs. Gesellsck. der Wissensch., Leipzig, 1850, S. 186. 



