306 PHYSIOLOGY. 



can be felt in the artery. The tambour at this time exhibits vibrations 

 due to the arterial pulse; but if the air-pressure in the cuff on the 

 arm be gradually lessened by means of an escape valve, these vibra- 

 tions become longer, and when this state of affairs ensues it shows 

 the pressure which the pulse-wave can just overcome, and is the 

 maximum systolic pressure. 



If the pressure be decreased still more the vibrations of the 

 tambour lever become larger until a point is reached at which they 

 begin to decrease. The pressure where the greatest fluctuations of 

 the arteries ensue causes the most extensive movements of the lever, 

 and is the minimum diastolic pressure. 



The majority of clinical sphygmomanometers give only the sys- 

 tolic pressure, and this pressure is the one usually meant when we 

 speak of a patient's blood-pressure in the clinic. But there is 

 also a diastolic pressure which depends upon the degree of contrac- 

 tion of the peripheral arterioles. Systolic pressure gives us a 

 knowledge of the force of the left ventricle, whilst diastolic pressure 

 teaches us the tone of the arteries. 



It must be remembered that fallacies exist in the use of such an 

 instrument. For the matter is only too evident that there will be 

 recorded compression of the venae comites of the artery, the skin 

 and surrounding tissues. Further, it is impossible to tell the exact 

 moment when the distal pulse is rendered imperceptible. 



Erlanger obtained in the brachial artery an average pressure of 

 110 millimeters during systole and 65 during diastole. He also found 

 that a heavy meal was followed by an increased output of blood by the 

 heart, as revealed by an increase in pulse-pressure. 



In the case of healthy, young adults, the pressure in the brachial 

 artery ranges between 110 and 130 millimeters of mercury. Pres- 

 sure attains its maximum with the individual in the erect position; 

 its minimum when he assumes the horizontal. 



Blood-pressure must always be sufficient to cause a steady flow 

 of blood through the capillaries, and this must be at least 50 milli- 

 meters of mercury. The blood-pressure which keeps up a head of 

 pressure above this to meet the varying calls of the organs for more 

 blood is about 70 millimeters. 



The blood-pressure remains at its normal height when the blood- 

 vessels receive an excess of fluid, or when the vessels contain less than 

 normal amount. When the excess of fluid arrives in the vessels it 

 remains in the central veins and in the liver, which becomes very hard. 

 This excess of fluid soon passes out of the vessels into the tissues. 



