382 



Prof. W. M. Bayliss. 



resistance in the peripheral arterioles and that this resistance is entirely due 

 to the internal friction or viscosity of the blood. The rate of flow is inversely 

 proportional to the viscosity and directly proportional to the driving pressure, 

 by Poiseuille's law. This law has been shown by Du Bois-Beyrnond, Brodie, 

 and Muller (1) to apply to the circulation, contrary to the objections of 

 Heubner and others. The output of the heart being constant, by hypothesis, 

 the rate of flow through the blood-vessels must also be constant, and therefore 

 if the viscosity of the blood decreases, the driving pressure must decrease 

 also. Otherwise, the current through the capillaries would be larger than 

 that supplied by the heart. In other words, the arterial pressure must fall. 

 Although this fact seems obvious, I thought it of interest to test it in a model 

 made of indiarubber tubing connected with a glass tube of - 97 mm. bore 

 and 27 cm. long. Through this a current of gum solutions of various con- 

 centrations and viscosities was driven by means of a small pump of constant 

 delivery of - 3 c.c. per stroke. The outflow from the capillary was found to 

 be constant, although the driving pressures ranged from 178 mm. of mercury, 

 when the viscosity was 3"7 times that of water, down to 45 mm. of mercury 

 with water itself. 



Now, the viscosity of blood is considerably higher than that of water or of 

 a dilute saline solution. In the cats used in my experiments it was usually 

 about three times that of water, but in one case it was only 2 - 2 times, and in 

 another it was as high as four times. The viscosity of Binger's solution, on 

 the other hand, is practically the same as that of water. It follows that, if 

 part of the blood is replaced by such a saline solution, the resulting 

 viscosity is correspondingly reduced, and, other things equal, the arterial 

 pressure must decrease. Thus, in one experiment, the viscosity was reduced 

 to l - 6 times that of water by replacing half of the blood by Binger's 

 solution. In another case it was reduced from 2'2 times to 1*8 times by the 

 replacement of 47 per cent, of the calculated blood volume by the saline 

 solution. 



The viscosities were determined at 38° in an apparatus similar to that described by 

 Scarpa (2), in which a known volume is driven first upwards through a capillary tube 

 by pressure and then allowed to run back downwards. The mean of several pairs of 

 readings was taken. The advantage of this method is that no correction need be made 

 for specific gravity or surface tension, since these factors correct themselves by acting 

 in opposite directions in upward and downward movement. 



The blood volume was taken to be 48 c.c. per kilogramme of body weight. 



In one experiment several samples of blood of 45 c.c. each were removed 

 and replaced by saline in series, so that the viscosity decreased each time. 

 The height of the arterial pressure, compared with the viscosity in each 

 stage, was as follows : — 



