VELOCITY AND PRESSURE OF BLOOD-FLOW. 



439 



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 ab : an :: 



AB:An, or ab = AB * an - 

 An 



According to this method, 



Vierordt calculates 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- 

 mum velocity being found in the vena cava. The general relations 

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



igrs 



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. 177. 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. 



may be expressed, therefore, by a curve such as is shown in Fig. 

 177. 



Explanation of the Variations in Velocity. The general rela- 

 tionship between the velocities in the different parts of the vascular 

 system is explained by the difference in the width of the bed in 

 which the blood flows. In the systemic circulation the main stem, 

 the aorta, branches into arteries which, taken individually, are smaller 



