VELOCITY AND PRESSURE OF BLOOD-FLOW. 503 



tambour horizontal while the pressure in the system is being lowered there 

 is 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 diastoHc pressure in the brachial artery, and also, 

 of course, the difference between the two, the pulse pressure, which may be 

 taken as an indication of the force of the heart-beat. 



The Normal Arterial 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 systoUc 

 pressures, for example. In such experiments 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. 



Erlanger reports that in the adult (twenty to twenty-five) , when 

 the psychical factor is excluded, the average pressure in the brachial 

 is 110 mms. systolic, and 65 mms. diastolic. Von Recklinghausen's 

 figures for the same artery are, systolic pressure 1 16 mms. Hg., dias- 

 tolic pressure 73 mms. Hg. In a series of observations made upon 

 college students between the ages sixteen to forty, and comprising 

 5807 women and 2930 men, Alvarez* reports that the average 

 systolic pressure for the men was 126.5 mms., and for the women 

 115 mms. A similar sex difference is reported by other observers. 

 A systolic pressure over 150 mms. is commonly regarded as evi- 

 dence of a pathological hypertension. 



Erlanger and Hooker report observations upon the effect of 

 meals, of baths, of posture, the diurnal rhythm, etc.j 



The effect of meals is particularly instructive in that it illustrates 

 admirably the play of the compensatory mechanisms of the circu- 

 lation by means of which the heart and the blood-vessels are ad- 

 justed to each other's activity. During a meal there is a dilatation 

 of the blood-vessels in the abdominal area, or, as it is frequently 

 called in physiology, the splanchnic area, since it receives its 

 vasomotor fibers through the splanchinc nerve. The natural 

 effect of this dilatation, if the other factors of the circulation 

 remained constant, would be a fall of pressure in the aorta and a 

 diminution in blood-flow to other organs, such as the skin and the 

 brain. This tendency seems to be compensated, however, by an 



* Alvarez, "Archives of Int. Med.," 26, 381, 1920. 



t Erlanger and Hooker, "The Johns Hopkins Hospital Report," vol. xii, 

 1904. 



