364 THE MECHANICS OF THE CIRCULATION, HEMODYNAMICS 



pressure throughout the venous system is very low and cannot therefore support a 

 column of mercury of adequate height nor deviate a membrane possessing shght 

 elastic powers. Furthermore, the venous pressure cannot be measured by con- 

 necting the manometer with the peripheral or central end of the vein, because the 

 blocking of the distal stump of the vein would give rise to a venous stagnation which 

 would be indicative of the pressure prevailing in the corresponding arterial supply 

 tubes. Quite similarly, the use of the central stump would expose the manometer 

 to the pressure existing in the more central vein. 



Having inserted a suitable cannula in the blood-vessel, the entire tubing 

 between it and the manometer is filled with a solution tending to prevent the 

 coagulation of the blood. A saturated solution of sodium carbonate or bicar- 

 bonate, a 5 per cent, solution of sodium citrate or a 25 per cent, solution of mag- 

 nesium sulphate may be used for this purpose. A device which often saves 

 much time is to connect the manometer with a reservoir containing one or the 

 other of these solutions, so that the connecting tubes may be flushed out when- 

 ever they become blocked by coagula. On the venous side, a 0.7 per cent, solu- 

 tion of sodium chlorid should be employed, because as the pressure encountered 

 in these channels is low, and may even fall below zero, a part of the fluid in the 

 connecting tube may be drawn into the circulation and, unless non-toxic, may 

 produce depressive effects. In some cases it may be necessary to render the 

 blood as a whole non-coagulable, which end may be accomplished by the injection 

 of a solution of peptone or of an extract of leeches (hirudin). 



On removing the clamp previously placed upon the artery, the blood will 

 be seen to enter the connecting tube and to displace the column of mercury out- 

 ward until the weight of the latter exactly counterbalances the blood pressure. 

 As soon as an equilibrium between these two opposing forces has been established, 

 the mercury undergoes a series of rhythmic fluctuations, the smaller ones of which 

 are dependent upon the contractions of the heart and the larger ones upon the 

 respiratory movements. The former are known as the cardiac and the latter as 

 the respiratory variations in the arterial blood pressure. Both must be sharply 

 differentiated from oscillations of a similar kind which appear in the central veins 

 and are designated as the cardiac and respiratory variations in venous pressure. 

 Moreover, if the experimental conditions are especially favorable, a third type of 

 variation frequently appears in the arteries which is of much longer duration 

 than the others and is known as the Traube-Hering curve. The character and 

 cause of these changes will be considered more fully in a subsequent chapter. 



It has been pointed out above that the mercury is quite unable to follow 

 quick changes in pressure with accuracy. On this account, a membrane manome- 

 ter should be used whenever it is desired to depict the character of the individual 

 ■pulsations. A mercury manometer, on the other hand, should be employed when- 

 ever it is intended merely to obtain a general picture of the height of the pressure. 

 Special directions for the use of these instruments have been given previously 

 (page 293). 



The Arterial Pressure in Different Animals and Arteries. — The 

 direct method has been applied to man in a few isolated cases, when 

 it became necessary in the course of operations to divide certain 

 peripheral blood-vessels. For the femoral and brachial arteries^ 

 the average value of 120 mm. Hg has been found and for the tibial 

 the value of 80-90 mm. Hg. The pressures obtained under the most 

 favorable conditions in other animals have been compiled by Volkmann 

 and Nikolai as follows: 



1 Faivre, Gazette m6d. de Paris, 1856, and Albert, Med. Jahrb., Wien, 1883. 



