THE CIRCULATION OF THE BLOOD. 343 



C. In Venous Pressure. Independent of any change in the venous 

 pressure- in a given area from local or temporarily acting causes e.g., aspira- 

 tion of the thorax or heart, muscle contractions, change of position, etc. the 

 general venous pressure will be increased by a decrease in the value of those 

 factors which produce the difference of pressure between the arteries and 

 veins. An increase in the value of these factors would necessarily decrease 

 the pressure. 



Variations in the Relation of the Arterial and Venous Pressures. 

 So long as the heart maintains a given rate and force and the resistance at 

 the periphery of the arterial system (due to the contraction of the arteriole 

 muscle) a given value, will the usual physiologic difference between the pres- 

 sure in the arteries and veins remain unchanged. If, however, either 

 factor changes in one direction or another, there will arise a change in the 

 relative degree of pressure in the different divisions of the vascular apparatus. 

 Thus if the heart force increases and a larger volume of blood is discharged 

 into the arteries in a unit of time, the amount of blood in the venous system 

 diminishes, and the result is a rise of the arterial and a fall of the venous 

 pressures. If, on the contrary, the heart force decreases or the mitral valve 

 permits of a regurgitation, a smaller volume of blood is ejected into the 

 arteries in a unit of time, the amount of blood in the venous system increases, 

 and the result is a fall of the arterial and a rise of the venous pressure. 



Again if the arteriole muscle relaxes and a larger volume of blood flows 

 from the arteries into the veins in a unit of time, the result will be a fall of 

 arterial and a rise of venous pressure. If, on the contrary, the arterial muscle 

 contracts and a smaller volume of blood flows into the veins, the reverse 

 change of pressure obtains. 



The Determination of the Arterial Blood-pressure in Man. Inas- 

 much as the blood-pressure undergoes considerable variation in both physi- 

 ologic and pathologic conditions as well as in response to the action of drugs, 

 it seemed desirable to possess some means by which an accurate knowledge 

 of the pressure under a variety of conditions could be obtained both for 

 diagnostic and therapeutic purposes. The foregoing method of obtaining 

 the blood-pressure not being of general application to human beings for 

 obvious reasons, special instruments have been devised by which the pres- 

 sures may be determined at least approximately without resorting to any 

 surgical procedure. These instruments are termed sphygmomanometers. 

 Some of the many forms of this instrument are adapted for obtaining the 

 systolic pressure only, while others are adapted for obtaining either the sys- 

 tolic or the diastolic pressure, or both. 



The principle involved in the first group is the application of a hydrostatic 

 pressure to an artery, e.g., the temporal, radial, etc., until the lumen is com- 

 pletely obliterated as indicated by the disappearance of the pulse beyond the 

 point of compression, and at the same time the registration of the pressure 

 applied, by means of a mercurial or spring manometer. The pressure just 

 sufficient to obliterate the pulse or to allow it to reappear after obliteration, 

 is taken as the systolic pressure. 



The principle involved in the second group is based on a suggestion of 

 Marey, that the maximum pulsation of the artery or the maximum distention 

 and recoil following a heart-beat would be most likely to take place when 



