VELOCITY AND PRESSURE OF BLOOD-FLOW. 457 



The float in turn bears an upright steel wire which at the end of the glass tube 

 plays through a small opening in a metal or glass cap. At its free end it bears 

 a pen to trace the record. If smoked paper is used the pen is simply a smooth- 

 pointed glass or metal arm, while if white paper is employed the wire carries 

 a small glass pen with a capillary tube, which writes the record in ink. The 

 tube connecting the proximal end of the manometer to the artery of the ani- 

 mal must be filled with a solution that retards the coagulation of blood. For 

 this purpose one employs ordinarily a saturated solution of sodium carbonate 

 and bicarbonate. This tube is connected also by a T piece to a reservoir con- 

 taining the carbonate solution, and by varying the height of this latter the 

 pressure in the tube and the mafnometer may be adjusted beforehand to the 



g-essure that is supposed or known to exist in the artery under experiment, 

 y this means the blood, when connections are made with the .manometer, 

 does not penetrate far into the tube, and clotting is thereby delayed. In 

 long observations it is most convenient to use what is known as a washout 

 cannula, the structure of which is represented in Fig. 181, B. When this 

 instrument is attached to the cannula inserted into the blood-vessel one can, 

 after first clamping off the artery, wash out the connections between the artery 

 and the manometer with fresh carbonate solution as often as desired. By 

 such means continuous records of arterial pressure may be obtained during 

 many hours. Determinations of the pressure in the veins may be made with a 

 similar apparatus, but owing to the low values that prevail on this side of 

 the circulation it is more convenient to use some form of water manometer 

 and thus record the venous pressures in terms of the height of the water column- 

 supported. It should be added also that when it is necessary to know the 

 pressure in any special artery or vein the connections of the manometer are 

 made usually to a side branch opening more or less at right angles into the 

 vessel under investigation, or if this is not possible then a T tube is inserted 

 and the manometer is connected with the side branch. The reason for this 

 procedure is that if the artery ifsell is ligated and the manometer is con- 

 nected with its central stump, the flow in it and its dependent system of capil- 

 laries and veins is cut off; the stump of the artery constitutes simply a con- 

 tinuation of the tube from the manometer and serves as a side connection 

 to the intact artery from which it arises. Thus, when a manometer is inserted 

 into the carotid artery the pressure that is measured is the side-pressure in 

 the innominate or aorta from which it arises, while a cannula in the central 

 stump of a femoral artery measures the pressure in the iliac. A specimen of 

 what is known as a blood-pressure record is shown in Fig. 182. The exact 

 pressure at any instant, in millimeters of mercury, is obtained by measuring 

 the distance between the base line and the record and multiplying by 2. 

 The base line represents the position of the recording pen when it is at ita 

 zero position for the conditions of the experiment. It is necessary to mul- 

 tiply the distance between the base line and the record by 2 because, as 

 is seen in Fig. 181, the recording apparatus measures only the rise of the 

 mercury in one limb of the manometer; there is, of course, an equal fall in the 

 other limb. 



The blood-pressure record (Fig. 182) shows usually large rhyth- 

 mical variations corresponding to the respiratory movements and in 

 addition smaller waves caused by the heart beat. The causes of the 

 respiratory waves of pressure are discussed in the section on respi- 

 ration. Regarding the heart waves or pulse waves the usual record 

 obtained by means of a mercury manometer gives an entirely false 

 picture of the extent of the variations in pressure caused by the heart 

 beat. The mass of mercury possesses considerable weight and iner- 

 tia, which unfits it for following accurately very rapid changes in 

 pressure. When the pressure changes are slow, as in the case of 

 the long, respiratory waves seen in the record, the manometer un- 



