76 



CIRCULATION OF THE BLOOD. 



the one now generally used. It consists of a small but thick glass bottle, with a fine, 

 graduated glass tube about twelve inches in length, communicating with it, either through 

 the stopper or by an orifice in the side. The stopper is pierced by a bent tube which is 

 to be connected with the blood-vessel. 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 amount of press- 

 ure on the mercury in the bottle will be indicated by an elevation in the graduated tube ; 

 and, moreover, from the fineness of the column in the tube, we avoid some of the in- 

 conveniences which are due to the weight of mercury in the hsemadynamometer, and we 

 also have less friction. This instrument is appropriately called the cardiometer, as it in- 

 dicates most accurately, by the extreme elevation of the mercury, the force of the heart; 



FIG. 25 (A.). Section of the cardiometer of Jfagendie, 

 as modified by Bernard. 



A strong glass bottle is perforated at each side and fitted 

 with an iron tube, with an opening, T, by which the 

 mercury enters. One end of the iron tube is closed, 

 and the other is bent upward and connected with 

 the graduated glass tube T', which has a caliber of 

 from T V to | of an inch. The bottle is filled with 

 mercury until it rises to n' in the tube which is 

 marked zero. The cork is perforated by the tube t, 

 which is connected by a rubber tube with the point 

 C, which is introduced into the vessel. 



FIG. 25 (B). Compensating instrument of Marey. 



but it is not as perfect in its indications of the mean arterial pressure, for, in the abrupt 

 descent of the mercury during the diastole of the heart, the impetus causes the level to 

 fall considerably below the real standard of the constant pressure. Marey has succeeded 

 in correcting this difficulty in what he calls the " compensating " instrument, which is 

 constructed on the following principle : Instead of a simple glass tube which communi- 

 cates with the mercury in the bottle, as in Magendie's cardiometer, he has two tubes, 

 one of which is like the one already described and represents oscillations produced by 

 the heart, while the other is larger, and has, at the lower part, a constriction of the 



