VELOCITY AND PRESSURE OF BLOOD-FLOW. 491 



The distal limb of the U tube in which the mercury rises carries a float 

 of hard rubber, aluminum, or some other substance lighter than the mercury. 

 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 or a 5 per cent., solution of sodium citrate. This tube 

 is connected also by a T piece to a reservoir containing the carbonate solu- 

 tion, and by varying the height of this latter the pressure in the tube and 

 the manometer may be adjusted beforehand to the pressure that is sup- 

 posed or known to exist in the artery under experiment. By this means 

 the blood, when connections are made with the manometer, does not pen- 

 etrate far into the tube, and clotting is thereby delayed. In long obser- 

 vations it is most convenient to use what is known as a washout cannula, 

 the structure of which is represented in Fig. 192, B. When this instru- 

 ment 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 itself 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. 193. 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 its 

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

 the distance between the base line and the record by 2, because, as is seen in 

 Fig. 191, 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. 193) 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 



