VELOCITY AND PRESSURE OF BLOOD-FLOW. 451 



rents of known velocity, so that the angle of deflection may be 

 expressed in terms of absolute velocities. It possesses the great 

 advantage over the stromuhr that it gives not simply the average 

 velocity during a given time, but also the variations in velocity 

 coincident with the heart beat or other changes that may occur 

 during the period of observation. 



Efforts have been made to Revise a method for the determination of the 

 velocity of the blood-flow in the arteries of man. The method used, however, 

 depends upon certain assumptions that are not entirely certain and the re- 

 sults obtained, therefore, can not be used with confidence. The principle of 

 the method consists * in determining the volume of the arm by placing it in 

 a plethysmograph. Assuming that the outflow from the veins is constant in 

 the part of the arm inclosed, then the variations in volume of the arm may 

 be referred to the greater inflow of blood into this part through the arteries. 

 The curve showing the variations in volume may, therefore, under proper 

 conditions, be interpreted in terms of velocity changes. 



Mean Velocity of the Blood-flow in the Arteries, Veins, and 

 Capillaries. Actual determinations of the average velocity in the 

 large arteries and veins give such results as the following: Carotid 

 of horse (Volkmann), 300 nuns, per second; (Chauveau) 297 mms. 

 Carotid of the dog (Vierordt), 260 mms. 



The flow in the carotid, as in the other large arteries, is not, 

 however, uniform; there is a marked acceleration or pulse at each 

 systole of the heart during which the velocity is greatly augmented. 

 Thus, in the carotid of the horse it has been shown by the hemo- 

 dromograph that during the systole the velocity may reach 520 

 mms. and may fall to 150 mms. during the diastole. It is found, also, 

 that this difference between the systolic velocity and the diastolic 

 velocity tends to disappear as the arteries become smaller, and, as 

 was said above, disappears altogether in the capillaries, in which 

 the pulse caused by the heart beat is lacking. The smaller the artery, 

 therefore, the more uniform is the movement of the blood. 



The flow in the large veins is approximately equal to that in 

 arteries of the same size. In the jugular vein of the dog, for 

 instance, Vierordt found a velocity of 225 mms., while in the 

 carotid of the same animal the average velocity was 260 mms. 

 In the capillaries, however, the velocity is relatively very small. 

 From direct observations made by means of the microscope and 

 from indirect observations in the case of man the capillary velocity 

 is estimated as lying between 0.5 mm. and 0.9 mm. per sec. 



Vierordt reports some interesting calculations upon the velocity of the 

 blood, in the capillaries of his own eye. Under suitable conditions,! the 

 movements of the corpuscles in the retina may be perceived in consequence 

 of the shadows that they throw upon the rods and cones. The visual images 



* Von Kries, "Archiv f. Physiologic," 1887, 279; also Abeles, ibid., 1892, 

 22. 



t "Archiv f. physiologische Heilkunde," 15, 255, 1856. 



