488 



CIRCULATION OF BLOOD AND LYMPH. 



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 

 thus produced may be projected upon a surface at a known distance from the 

 eye and the space traversed in a given time may be observed. The distance 

 actually covered upon the retina may then be calculated by the following con- 

 struction, in which A-B = the distance traveled by the projected image; 

 A-n, the distance of the surface from the eye; and a-n. the distance of the 



retina from the nodal point 

 of the eye. We have then 

 the proportion db : an :: 



AB:An, or ab = AB * an - 



An 



According to this method, 

 Vierordt calculated that the 

 velocity of the blood in the 

 human capillaries is equal to 

 about 0.6 to 0.9 mm. per 

 second. 



In the arteries, more- 

 over, it may be observed 

 that the average velocity 

 diminishes the farther 



one goes from the heart, that is, the smaller the artery, and 

 reaches its minimum when the arteries pass into the capillaries. 

 Thus, Volkmann reports for the horse the following figures: Ca- 

 rotid, 300 mms.; maxillary, 232; metatarsal, 56 mms. In the veins 

 also the same fact holds. The smaller the vein that is, the nearer 

 it is to the capillary region the smaller is its velocity, the maxi- 



Fig. 190. Diagram of the eye to show the con- 

 struction used to determine the size of the retinal 

 image when the size of the external object is known: 

 n, The nodal point of the eye. See text. 



Fig. 191. Schematic representation of the relative velocities of the blood-current in 

 different parts of the vascular system: a, The arterial side, indicating the changes with 

 each heart beat and the fall of mean velocity as the arterial bed widens; c, the capillary 

 region the great diminution in velocity corresponds with the great widening of the bed ; 

 v, the venous side, showing the gradual increase toward the heart, and represented as 

 entirely uniform, although, as a matter of fact, the velocity in the large veins is affected by 

 the respirations and to a small extent by the heart beat, owing to the phenomenon known 

 as the venous pulse (p. 532). 



mum velocity being found in the vena cava. The general rela- 

 tions of the velocity of the blood in the arteries, capillaries, and 

 * "Archiv f. physiologische Heilkunde/' 15, 255, 1856. 



