VELOCITY OF THE BLOOD CURRENT. 315 



the rate of the blood flow which is caused by the heart beat 

 diminishes with the force of the pulse as the smaller arteries 

 are approached, and finally ceases completely in the capillaries, 

 where, under ordinary circumstances, the flow is perfectly con- 

 tinuous. In the first part of the aorta the velocity of the blood 

 flow is reduced to nil after each ventricular beat, while in the 

 capillaries no change is perceived. Between these two extremes 

 all gradations may be found, which follow the same rules as the 

 pulse. 



The general mean velocity varies directly with the blood 

 pressure, which bears an inverse relation to the calibre of the 

 arteries, and further, the mean velocity in any one artery, and 

 its branches, will vary with the diameter of the vessels, which 

 are constantly undergoing local changes in size. 



Generally speaking, quick heart beats cause increase in velocity 

 of the stream, but no definite or invariable relation exists between 

 the two, the vasomotor influences having, no doubt, much more 

 effect than the heart beat on the rate of the stream in the smaller 



In looking at the blood passing through the small vessels of 

 a transparent tissue, such as the frog's tongue or web, it appears 

 that different parts of the column of fluid move with different 

 velocities. Down the centre of the stream the red corpuscles 

 are seen coursing rapidly, while between the central part and the 

 vessel wall on each side a pale line of plasma can be recognized, 

 which seems to flow more slowly and to carry with it only a few 

 white corpuscles. 



In the veins the velocity varies enormously with a variety of 

 circumstances which have little or no effect on the arterial flow. 

 Thus, the position of the body or limb, the activity of the neigh- 

 boring muscles and the respiratory movements alter it, but, as a 

 general rule, the flow in the veins is pretty steady, there being no 

 pulsation or corresponding variation of velocity. In the large 

 vessels the onward flow is affected by the contraction of the auri- 

 cles. During the auricular systole the veins cannot empty them- 

 selves, and therefore there is a slight check to the onward flow, 

 and the velocity of the current is accordingly reduced. In cases 



