ON THE FUNCTIONS OF THE HEART AND ARTERIES. gj 



twentieth division, at the distance of an inch and a quarter cept in the mi- 

 only frora the extieme capillary arteries, the pressure of a ""'^^''*^« s- 

 column of one twentieth of an inch only is required for 

 overcoming the whole friction, and at the twenty-fifth divi- 

 sipn, where the artery does not much exceed the diameter 

 of a human hair, the height to which the water would rise, 

 in a tube fixed laterally into the artery, is only two inches 

 less than in the immediate neighbourhood of the heart. 



In order to judge of the comparative resistance produced Resistance of 

 by fluids of different degrees of viscidity, I employed the fes3^^,']2y7 *^ 

 same tubes, by means of which I had determined the fric- 

 tion of water, in extreme cases, for ascertaining the effect of 

 different substances held in solution in the water: since 

 it is impossible to make direct experiments on the blood 

 in its natural state, on account of its tendency to coagulate; 

 and those substances, which have the power of preventing 

 its coagulation, may naturally be supposed to produce a 

 material change in its viscidity. The diameter of one of 

 the tubes, which was cylindrical, was the fortieth part of 

 an inch: the bore of the other was oval, as is usual in the 

 finest tubes made for thermometers: the section, divided 

 by one fourth of the circumference, gave one hundred and 

 seventy seconds for the mean diameter. I caused some 

 milk, and solutions of sugar of different strength, to pass 

 through these tubes : they were all transmitted much more 

 sparingly than water, with an equal pressure, and the dif- 

 ference was more considerable in the smaller than in the 

 larger tube, as might naturally be expected, both from the 

 nature of the resistance, and from the result of Gerstner's ex- 

 periments on water at different temperatures. In the first 

 tube the resistance to the motion of milk was three times as 

 great as to that of water, a solution of sugar in five times 

 its weight of water produced twice as much resistance as 

 water; in twice its weight, nearly four times as much as 

 water: but in the narrower tube, the weaker solution of 

 sugar exhibited a resistance five times as great as that of 

 water, which is more than twice as much as appeared in the 

 larger tube. Hence there can be no doubt, that the resist- Calculatioa 

 tance of the internal surface of the arteries to the motion for blood. 

 of the blood must be much greater, than would be found 



in 



