VELOCITY AND PRESSURE OF BLOOD-FLOW. 471 



the moment when the pulse wave is first able to break through the brachial 

 artery, and it gives, therefore, the systolic pressure. After finding the sys- 

 tolic pressure the diastolic pressure is obtained by allowing the pressure to 

 drop still further. The pulsations increase in height to a maximum size and 

 then decrease. The pressure at which the maximum pulse wave is obtained 

 marks the diastolic pressure. It is better perhaps in dropping the pressure 

 for this last purpose to manipulate stopcock i so as to drop the pressure 5 

 mms. at a time, recording the pulse wave at each pressure. In this way a 

 record is obtained such as is given in Fig. 193. It should be added, also, that 

 in order to keep the lever of the tambour horizontal while the pressure in the 

 system is being lowered there i a minute pinhole in the metal bottom of the 

 tambour. Through this pinhole the pressure in the tambour and chamber, 

 /, is kept atmospheric throughout, except during the quick changes caused 

 by the pulse waves. By means of this instrument one can determine within 

 a minute or so the amount of the systolic and diastolic pressure in the brachial 

 artery, and also, of course, the difference between the two, the pulse pressure, 

 which may be taken as a measure of the force of the heart beat. 



The Normal Pressure in Man and its Variations. By means 

 of one or other of the instruments devised for the purpose numerous 

 results have been obtained regarding the blood-pressure in man at 

 different ages and under varying normal and abnormal conditions. 

 Unfortunately the methods used have not always been complete. 

 Some authors give only systolic pressures, for example. In such ex- 

 periments also a troublesome factor is always the psychical element. 

 The mental interest that the individual experimented upon takes in 

 the procedure almost always causes a rise of pressure and perhaps 

 a changed heart rate. Results, as a rule, upon any individual show 

 lower values after the novelty of the procedure has worn off and 

 the patient submits to the process as an uninteresting routine. 

 Under normal conditions Potain * estimated the systolic pressure in 

 the radial of the adult at about 170 mms. of mercury and the varia- 

 tions for different ages he expressed in the following figures: 



Age 6-10 15 20 25 30 40 50 60 80 



Pressure (systolic) 89 135 150 170 180 190 200 210 220 



Without the other side of the picture that is, the diastolic pres- 

 sure and the force of the heart beat (pulse pressure) it is difficult 

 to interpret these figures. The rapid increase up to maturity 

 probably represents chiefly the larger output of blood from the heart; 

 the slower and more regular increase from maturity to old age is 

 due possibly to the gradual hardening of the arteries, since the less 

 elastic the arteries become, the greater will be the systolic rise with 

 each heart beat. With his more complete apparatus Erlanger 

 reports that in the adult (20 to 25), when the psychical factor is 

 excluded, the average pressure in the brachial is 110 mms., systolic, 

 and 65 mms., diastolic, figures much lower than those given by 

 * "La pression arterielle de 1'homme," Paris, 1902. 



