ARTEKIAL PRESSURE. 265 



the hemodynamometer was made by Magendie. This ap- 

 paratus, the cardiometer^ in which Bernard has made some 

 important modifications, is 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 pressure 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 inconveniences which are due to 

 the weight of mercury in the hemodynamometer, and also 

 have less friction. 



This instrument is appropriately called the cardiometer, 

 as it indicates most accurately, by the extreme elevation of 

 the mercury, the force of the heart ; but it is not as perfect 

 in its indications of the mean arterial pressure, as in the ab- 

 rupt 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 " compensat- 

 ing " instrument ; which is constructed on the following prin- 

 ciple : Instead of- a simple glass tube which communicates 

 with the mercury in the bottle, as in Magendie's cardiometer, 

 he has two tubes : one of which is like the one already describ- 

 ed, and represents oscillations produced by the heart ; the 

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

 the caliber which is there reduced to capillary fineness. This 

 tube is designed to give the mean arterial pressure. The 



of the pencil, brought in contact with a revolving cylinder covered with paper, 

 produced a trace of the oscillations. By analysis of this trace he arrived at the 

 mean pressure in the arteries. This instrument was called the kymographion. It 

 has never been much used in investigation, and is entirely superseded by the car- 

 diometer of the present day. 



