PRESSURE OF BLOOD IN THE ARTERIES . 57 



five feet (1.5 meters) and remain at this point almost stationary, indicat- 

 ing, by a slight pulsatile movement, the action of the heart. Carefully 

 watching the level in the tube, in addition to the rapid oscillation coinci- 

 dent with the pulse, another oscillation will be observed, which is less 

 frequent and which corresponds with the movements of respiration. -The 

 pressure, as indicated by an elevation of the liquid, is_jslightly increased 

 during expiration and diminished during inspiration. In such experi- 

 * ments, it is necessary to fill part of the tube, or whatever apparatus is 

 used, with a solution of sodium carbonate, in order to prevent coagula- 

 tion of the blood as it passes out of the vessels. The experiment with 

 the long tube gives, perhaps, the best general idea of the arterial pres- 

 sure, which is equal to about five feet of blood (.1.5 meters) or a few inches 

 more of water. The oscillations produced by the contractions of the 

 heart are not very extensive, on account of the friction in so long a tube ; 

 but this is favorable to the study of the constant pressure. 



The experiments of Hales were made with a view of calculating the 

 force of the heart and were not directed particularly to the modifications 

 and variations of the arterial pressure. It is only since the experiments 

 performed by Poiseuille with the hemodynamometer, in 1828, that physi- 

 ologists have had reliable data on this latter point. Poiseuille's instru- 

 ment for measuring the force of the blood is a graduated U-tube, half 

 filled with mercury, with one arm bent at a right angle, so that it can 

 easily be connected with the artery. The pressure of blood is indicated 

 by a depression in the level of the mercury on one side and a correspond- 

 ing elevation on the other. This instrument possesses certain advan- 

 tages over the long glass tube ; but for estimating simply the arterial 

 pressure, it is much less useful, as it is more sensitive to the impulses of 

 the heart. For the study of the cardiac pressure, it has the disadvan- 

 tage, in the first place, of considerable friction, and again, the weight of 

 the column of mercury produces an extent of oscillation by mere impe- 

 tus, greater than that which would actually represent the alternation of 

 systole and diastole of the heart. 



An important improvement in the hemodynamometer was made by 

 Magendie. This apparatus, the cardiometer, in which Bernard made 

 some modifications, is the one now commonly used. It consists of a small 

 but thick glass bottle, with a fine graduated tube about twelve inches 

 (30.5 centimeters) in length, communicating with it, either through the 

 stopper or an orifice in the side. The stopper is pierced with a bent 

 tube which is to be connected with the bloodvessel. The bottle is filled 

 with mercury so that it will rise in the tube to a point which is marked 

 zero. It is evident that the pressure on the mercury in the bottle will be 

 indicated by an elevation in the graduated tube ; and, moreover, from 



