66 



THE MECHANISM OF THE CIRCULATION. 



a coefficient depending on the viscosity and temperature of the fluid, but 

 independent of the structure of the tube wall so long as this is wetted by the 

 fluid ; k is known as the transpiration coefficient. 



In such a model as is shown in Fig. 42, the velocity head can be easily 

 determined by an instrument which can be directly applied to the circulation. 1 

 The instrument is a glass tube (nqs, Fig. 43). 



" From n to q the tube is of a fine bore, and at n there is a small bell-shaped 

 opening. When the tube is plunged into moving liquid, so that the orifice (n) 

 faces the current, as at I (Fig. 42), the fluid will rise to the height lb, due to 

 velocity and pressure ; if n be turned at right angles to the current, the height 

 will be la ; and if turned so that n is with the current as at J, the height Je 

 will be less than the pressure head by an amount ce, which is nearly equal 

 to the velocity head. By means of this instrument very accurate results can 

 be obtained, especially for velocity, as, by taking the difference of two 



heights, the effect of capillarity 

 disappears. When the pres- 

 sure is high, as in an artery, 

 the following modification can 

 be adopted. The end of the 

 tube (s) is connected with a 

 rubber tube (P) provided with 

 a clamp. P is, in its turn, 

 connected with a glass reser- 

 voir (D) containing air ; this is 

 covered thickly with cotton- 

 wool, to prevent changes of sur- 

 rounding temperature affecting 

 the air. The glass reservoir 

 is connected with a mercury 

 manometer (M) and a syringe 

 by tubes (R and Q) provided 

 with clamps. The tube (P) 

 is clamped, and the pressure 

 is raised in the reservoir to 

 a point which is known to be 

 about that of the arterial tension. The tube.(nq) is then introduced into the 

 artery, and the clamp (Q) is closed and P opened." 



The tube (nq), in experiments with this instrument, is filled with a solution 

 of oxalate of potash to prevent clotting. In one of NicolPs researches, the 

 inferior mesenteric artery was exposed and divided in a dog, and through it 

 the end (n) was introduced into the abdominal aorta. At the beginning of the 

 experiment, the height of the fluid in ns indicated a pressure of 145 mm. Hg. 

 The velocity head was a little over 6 mm. Hg, indicating a mean velocity 

 = \/l2|7 = 346 mm., for V'- = 2gh. The change of height in ns due to pulsation 

 was 9 mm. Hg. 



The flow in a tube of varying diameter. — Since fluid is incompres- 

 sible, an equal amount must flow in the unit of time through every section of 

 the tube, and thus the velocity in any part of a tube which varies in diameter 

 stands in inverse proportion to the sectional area. In such a tube the pressure 

 gradient is steepest in the narrowest section, for there the velocity and the 

 friction is greatest. In two sections of equal diameter the pressure gradient is 

 the same. Where a wide section follows upon a narrower, the lateral pressure 

 may either sink, remain unaltered, or even rise. How this can be so will appear 

 from the following considerations. At any point of the tube the whole pressure 

 head (H) equals the sum of the velocity head (///) and the resistance head (h 2 ). 



Fig. 43. 



1 Nicolls, Journ. Physiol., Cambridge and London, 1896, vol. xx. p. 417. 



