VII 



MECHANICS OF THE HEAET 



189 



as elucidated by Csradini, is founded on simple laws of hydro- 

 dynamics. The engineer Darcy (1857) demonstrated that fluids 

 passing through a tube are not under the same pressure, nor do 

 they move with the same velocity, in the different lines into which 

 the fluid cylinder can theoretically be analysed. The velocity is 

 greatest, the pressure lowest, in the axial threads ; in the peripheral 

 lines the velocity is 

 lowest and the pres- 

 sure greatest. This 

 law was confirmed a 

 year later by the re- 

 searches of Ludwig, 

 who employed new 

 and ingenious meth- 

 ods. In order to esti- 

 mate the different 

 velocities of the lines 

 of current, he em- 

 ployed opaque gran- 

 ules of lycopodium 

 powder, suspended in 

 water running through 

 a glass tube. To esti- 

 mate the difference in 

 pressure, he observed 

 the direction of the 

 current in a bent tube, 

 introduced into the 

 main tube in such a 

 way that one aperture 

 was just inside the 

 wall of that tube, 

 whilst the other pro- 

 jected farther towards 

 the axis of the current. 

 Ceradini further 

 showed that when a 

 piston was pushed in- 

 to a cylindrical glass 

 tube, held in the ver- 

 tical position, and half filled with water in which lycopodium 

 seeds are suspended, a centripetal eddy may be observed at the 

 base of the fluid cylinder depressed by the piston, and a centri- 

 fugal eddy at the other end. When the movement of the piston 

 ceases, the water cylinder breaks up momentarily into an inner 

 cylinder, which continues to move forward, and an outer ring, 

 which moves backward, united by a centripetal eddy. 



FIG. 57. Apparatus for demonstrating mechanism of semi- 

 lunar valves. B, liulb of art. pulmoriaris from heart of 

 recently killed pig; J's, sinus of Valsalva ; S, Riidinger's 

 speculum, closed at one end by a simple glass plate ; 1', 

 elastic rubber syringe, with which systole and diastole can 

 be imitated by hand ; fa, valve by which water can be 

 aspirated in diastole from a receiver, and then raised, 

 during systole, to the constant level of an upper vessel fixed 

 at a height which corresponds to the mean blood pressure 

 of the pulmonary artery in the living animal. 



