CIRCULATION OF THE BLOOD. 531 



velocity at any point in the blood-vessel system, since measurements 

 show that at any one point the velocity is subject to very great varia- 

 tions. 



The velocit}* and pressure of the blood at any given point do not 

 correspond, and may even be in inverse ratio ; thus many causes, such 

 as obstruction of any part of the vascular system, will increase the blood 

 pressure at that point and decrease the velocity. As a rule, the pressure 

 at any point depends upon the distance of that point from the heart, 

 whether in the arterial or venous S3 r stem, while the velocity depends upon 

 the capacity of the vessels at that point. 



Where the area of the circulatory system is very large, as in the 

 capillaries, the blood circulates slowly, just as the current of a stream 

 becomes retarded as it widens into a lake. 



Various methods have been employed for calculating the rapidity of 

 the circulation in different blood-vessels. The following represents the 

 estimations as to the flow in the arteries, capillaries, and veins of the 

 horse (Volkmann): 



Carotid artery, 300 mm. per second 



Maxillary artery, 165 



Metatarsal artery, . . . . .56 



Capillaries, 0.5 to 0.8 



Jugular vein, ...... 100 



Vena cava, . . . . . .110 



Chauveau estimates the velocity of the blood-flow in the carotid of 

 the horse as varying from 520 to 150 mm. per second, the highest velocity 

 coexisting with the systole of the ventricle and the lowest with its dias- 

 tole. In the larger veins respiration also produces considerable varia- 

 tion in the velocity of the flow, the velocity being increased in inspira- 

 tion and decreased in expiration. The velocity of the circulation through 

 any one vessel may be modified by a number of causes ; provided the 

 artery maintains its calibre unchanged, the velocity would evidently be 

 dependent upon the propelling force. Therefore, an increase in the 

 energy of the heart's contraction, the calibre of the arteries remaining 

 unchanged, will produce an accelerated flow through those vessels, while 

 a decrease in the heart's energy will correspondingly retard the arterial 

 current. On the other hand, the heart's energy remaining the same, a 

 dilatation of the artery will cause a slowing of the current, and a reduc- 

 tion in the calibre of the artery will cause the current to become acceler- 

 ated. So, also, if the resistance to be overcome in the circulation of the 

 blood be reduced, as by the relaxation of the capillaries, the energy of the 

 heart's contraction and the calibre of the artery remaining the same, the 

 velocity of the blood-current will be increased ; while, again, an increase 

 in the resistance, the other conditions being unchanged, will retard the 

 blood-flow. 



