234 LECTURES TO SCIENCE TEACHERS. 



artery and that of the atmosphere may of course be judged 

 of by the difference between the heights of the two 

 columns. The instrument Poiseuille used was called a 

 Hsemadynamometer, because it was supposed to measure 

 the force of the circulation. By means of it he discovered 

 that the arterial pressure varied in different creatures, 

 partly according to the size, but that it was tolerably 

 constant in the same animal when in health ; and he 

 inferred, from measurements made on the higher animals, 

 that in man it amounts to about eight inches of mercury. 



This discovery was one of practical as well as theoretical 

 value, for the maintenance of good pressure in the arteries 

 is as essential to healthy life as the maintenance of a 

 proper temperature. There is no acute disease in which 

 our arterial pressure is not disturbed, scarcely a remedy 

 or poison of which the action does not more or less depend 

 on the effect which it exercises on the state of distension 

 of the blood vessels. 



It was for the investigation of the variations of arterial 

 pressure that the graphic method was first applied by 

 Ludwig. The instrument first used by him for the purpose 

 differs from that of Poiseuille, only in this respect that a 

 contrivance is added to it by which it is converted into a 

 graphical instrument. On the surface of the mercury 

 column in the open limb of the U tube a light float, made 

 of black caoutchouc, rests. To this float is connected a 

 stem of the same material which near its upper end is 

 crossed by a horizontal pencil (Fig. 3). I will now com- 

 municate an oscillatory movement to the mercury column 

 on which the float rests, by working a syringe, of which the 

 interior is in communication with the other limb of the (J 

 tube. You will see that every motion of the mercury 

 column will be followed by the float, and accurately re- 

 corded on the cylinder by the pencil. 



Here is another manometer constructed on the same 

 principle, which I have put on the table simply for the 

 reason that it was the first ever made in this country. The 

 stem of the float, instead of having a pencil attached to it 

 directly, supports a long, carefully-counterpoised lever, by 

 which, in a manner that I shall explain more fully after- 

 wards, its motions are amplified. I exhibit to you tracings of 

 the variations of arterial pressure made with this machine. 



