PRESSURE OF BLOOD IN THE ARTERIES. 



75 



column of blood which could be sustained by the arterial pressure. In some experiments 

 on the carotid of the horse, the blood mounted to the height of from eight to ten feet. 



All experiments on the arterial pressure are made 

 on' the principle of the experiment of Hales, which, 

 with reference simply to the constant pressure in the 

 arteries, is as useful as those of later date and much 

 more striking. The only inconvenience is in the ma- 

 nipulation of the long tube ; but this may be avoided 

 by setting it in a strip of wood, when it can be easily 

 handled. If a large artery, as the carotid, be exposed 

 in a living animal, and a metallic point, connected 

 with a vertical tube of small caliber and from seven 

 to eight feet long by a bit of elastic tubing, be secured 

 in the vessel, the blood will rise to the height of about 

 six feet and remain at this point almost stationary, 

 indicating, by a slight pulsatile movement, the action 

 of the heart. On carefully watching the level in the 

 tube, in addition to the rapid oscillation coincident 

 with the pulse, another oscillation will be observed, 

 which is less frequent and which corresponds with 

 the movements of respiration. The pressure, as indi- 

 cated by an elevation of the fluid, is slightly increased 

 during expiration and diminished during inspiration. 1 



The experiment with the long tube gives us the 

 best general idea of the arterial pressure, which will 

 be found to vary between five and a half and six feet 

 of blood, or a few inches more of water. The oscil- 

 lations produced by the contractions of the heart are 

 not very marked, on account of the great friction in 

 so long a tube ; but this is favorable to the study of 

 the constant pressure. It has been found that the 

 estimates above given do not vary very much in ani- 

 mals of different sizes. Bernard found the pressure in the carotid of a horse but little 

 more than in the dog or rabbit. In the larger animals, it is the force of the heart which 

 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 conditions and variations of the arterial 

 pressure. It is only since the experiments performed by Poiseuille with the hfflinadyna- 

 mometer, in 1828, that we have any reliable data on this latter point. Poiseuille'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 19 

 generally considered as possessing great advantages 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 disadvantage, 

 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 which 

 would actually represent the force of the heart. 



An important improvement in the hcemadynamometer was made by Magendie. This 

 apparatus, the cardiometer, in which Bernard has made some important modifications, is 



i In all these experiments on the arterial or cardiac pressure, it is necessary to fill part of the tube, or whatever 

 apparatus we may use, with a solution of carbonate of soda, in order to prevent coagulation of the blood as it passes 

 out of the vessels. 



FIG. < i.na>madynamometer qfPoiseuille, 

 modified by Ludwig, Spengler, and 

 Valentin. 



The instrument is connected with the ves- 

 sel V V, in such a manner that the 

 circulation is not interrupted. The ele- 

 vation of the mercury in the branch B C 

 indicates the amount of pressure. 



