VELOCITY OF THE BLOOD CURRENT. 309 



The following table may be useful in giving a general idea of 

 the average velocity in different parts of the circulation : 



Near valves of aorta while the ventricles are con- 

 tracting it reaches 1200 mm. per sec. 

 Descending aorta, . . 300-600 

 Carotid, .... 205-357 



Radial, 100 mm. per sec. 



Metatarsal, .... 57 " 



Arterioles, .... 50 

 Capillaries, .... .5 

 Venous radicles, ... 25 

 Small veins on dorsum of hand, 50 " 



Vense cavse, .... 200 



In the aorta near the valves the blood current varies in rapidity, 

 because the flow through the aortic orifice is intermittent, and 

 this variation must be more or less communicated to the neighbor- 

 ing arteries in the form of an increase of rapidity coincident with 

 the beat of the arterial pulse. The variation in 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 continuous. 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 per- 

 ceived. 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 pres- 

 sure, 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 vaso-motor influences having, no doubt, much more 

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

 vessels. 



