CHAP, iv.] THE VASCULAR MECHANISM. 175 



Now, changes in the volume of the arm are mainly caused (we may 

 for the present neglect other causes) by changes in the quantity of 

 blood present in that portion of the arm which lies within the cylinder. 

 Upon examination it is found that besides certain slower changes of 

 volume which take place from time to time, there are changes of volume 

 corresponding to each heart beat. At each heart -beat the volume first 

 increases and then decreases again, reaching before the next heart beat 

 the same measure which it had just preceding the beat ; there is, we 

 may say, a pulsation of volume like the actual pulse ; and we may, by 

 the graphic method, obtain a curve of the changes in volume, a " volume 

 curve." An increase of volume, a rise of the curve, means that the 

 blood is flowing into the arm, within the cylinder, by the (axillary) 

 artery at the level of the rim of the cylinder, more swiftly than it is 

 flowing out by the (axillary) vein or veins at the same level ; a decrease 

 of volume, a fall of the curve, means that the blood is flowing in less 

 swiftly than it is flowing out ; and a stationary volume, the curve 

 neither rising nor falling, means that the blood is flowing in just as fast 

 as it is flowing out. The steeper the ascent of the volume curve, the 

 greater is the rapidity of the arterial inflow, and any lessening of the 

 steepness of the ascent means a diminution of that rapidity ; when 

 the steepness is lessened so much that the curve runs parallel to the 

 base line, then, whatever the actual height of the curve, the inflow by 

 the artery is only just as rapid as the outflow by the vein. Hence, the 

 dimensions of the parts of the apparatus being known, we may calculate 

 how many more or how many less cubic cm. of blood are flowing per 

 second, or per fraction of a second, in by the artery, than are flowing 

 out by the vein. But, as we have seen, the flow in the veins is constant 

 so far as each individual heart beat is concerned : it is not directly 

 influenced by each heart beat. Hence, having obtained by means of 

 the instrument a curve of the change of volume of the arm, we may 

 from that calculate out a curve of the changes in rapidity of the flow 

 in the artery at the level of the mouth of the cylinder. In this 

 way it is ascertained that with each heart beat the rapidity of the flow 

 at first rises very quickly, then more slowly, then ceases to rise, after 

 which it sinks, and, indeed, sinks to such a degree as to shew that 

 the blood at this moment is flowing less rapidly in the artery than in 

 the vein, but subsequently rises again to fall once more, just before the 

 next heart beat, to the same rate as at the beginning of the beat which 

 is being studied. Moreover, it is possible by help of certain assump- 

 tions to calculate the amount of the whole flow through the artery 

 (and through the vein) in a given time, that is to say, the actual 

 rapidity of the flow. 



In the capillaries, the rate is slowest of all. In the web of the 

 frog the flow as judged by the movement of the red corpuscles may 

 be directly measured under the microscope by means of a micro- 

 meter, and is found to be about half a millimeter in a second ; 

 but this is probably a low estimate, since it is only when the 

 circulation is somewhat slow, slower, perhaps, than what ought to 

 be considered the normal rate, that the red corpuscles can be 

 distinctly seen. In the mammal the rate has been estimated 



