382 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



when dry fixes the trace, which thereafter will not be spoiled by handling. 

 In Figure 101 the uppermost line shows a trace which fairly represents the 

 successive fluctuations of the aortic pressure of the dog. The longer and 

 ampler fluctuations are respiratory, the briefer and slighter are cardiac. In 

 each respiratory curve the lowest point and the succeeding ascent coincide with 

 inspiration ; the highest point and the succeeding descent with expiration. 

 The horizontal middle line is the base line, representing the pressure of the 

 atmosphere. The base-line has been shifted upward in the figure simply in 

 order to save room on the page. In the lowermost line the successive spaces 

 from left to right of the reader represent successive intervals of time of two 

 seconds each, written by an electro-magnetic chronograph. The pressure-trace 

 taken from a vein may in certain regions near the chest show respiratory fluc- 

 tuations, but nowhere cardiac ones, as the pulse is not transmitted to the veins. 

 The venous pressure is so small, that for the practical study of it a recording 

 manometer must be used in which some lighter fluid replaces the mercury, 

 which would give a column of insufficient height for working purposes. The 

 values obtained are then reduced by calculation to millimeters of mercury, for 

 comparison with the arterial pressure. The intravascular pressure at a given 

 moment can be measured by measuring a vertical line or " ordiuate " drawn 

 from the curve written by the manometer to the horizontal base-line. The 

 latter represents the height of the mauometric column when just disconnected 

 from the blood-vessel ; that is, when acted upon only by the weight of the 

 atmosphere and of the solution of sodium carbonate. To ascertain the blood- 

 pressure, the length of the line thus measured must be doubled ; because the 

 mercury in the proximal limb of the manometer sinks under the blood-pres- 

 sure exactly as much as the float rises in the distal limb. A small correction 

 must also be made for the weight of the solution of sodium carbonate. 



The Mean Pressure. The " mean pressure " is the average pressure dur- 

 ing whatever length of time the observer chooses. The mean pressure for the 

 given time is ascertained from the manometric trace by measurements too 

 complicated to be explained here. As the weight and consequent inertia of 

 the mercury cause it to fluctuate according to circumstances more or less than 

 the pressure, the mean pressure is much more accurately obtained from the 

 mercurial manometer than is the true height of each fluctuation, which is very 

 commonly written too small. Therefore, it is especially the mean pressure 

 that is studied by means of the mercurial manometer. The true extent and 

 finer characters of the single fluctuations caused by the heart's beat are better 

 studied with other instruments, as we shall see in dealing with the pulse. 



It has been seen that the blood flows continuously through the capillaries 

 because the pressure is continually high in the arteries and low in the veins. 

 The reader is now in position to understand statements of the blood-pressure 

 expressed in millimeters of mercury. The mean aortic pressure in the dog is 

 far from being always the same even in the same animal. We have found it, 

 in the case referred to on page 377, to be equivalent to about 121 millimeters 

 of mercury. It will very commonly be found higher than this, and may range 



