72 



CIRCULATION OF THE BLOOD IN THE VESSELS. 



is increased, and not, to any considerable extent, the constant pressure in the 

 vessels. 



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 hfemadynamometer, in 1828, that 

 physiologists have had any reliable data on this latter point. Poi- 

 seuille's instrument for measuring the force of the blood is a simple, 

 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 the blood is indicated by a depression in the level of the 

 mercury on one side and a corresponding elevation on the other. 

 This instrument is generally considered as possessing great 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 impulse 

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

 advantage, in the first place, of considerable friction, and again, the 

 weight of the column of mercury produces an extent of oscillation 



by its mere impetus, greater than that 

 e c which would actually represent the alter- 



nation of systole and diastole of the heart. 

 An important improvement in the 

 haemadynamometer was made by Magen- 

 die. This apparatus, the cardiometer, in 

 which Bernard made some important mod- 

 ifications, is the one now generally used. 

 It consists of a small but thick glass bot- 

 tle, with a fine, graduated glass tube about 

 twelve inches (3O5 centimetres) 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 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, some of the inconveniences which are due to the weight of mercury 

 in the hsemadynamometer are avoided, and there is, also, 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, for in 

 the abrupt descent of the mercury during the diastole of the heart, the im- 

 petus causes the level to fall below the real standard of the constant pressure. 

 Marey has corrected this difficulty in the " compensating " instrument, which 



FIG. 28. Section of the cardiometer of 

 Magendie, 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 graudated glass 

 tube T', which has a caliber of T V to 

 J of an inch (21 to 3'2 mm.). The bottle 

 is filled with mercury m, until it rises 

 to ' in the tube, which is marked zero. 

 The cork is perforated by the tube t, 

 which is connected by a rubber tube e 

 with the point C, which is introduced 

 into the vessel. 



