THE VASCULAR MECHANISM. 



189 



Velocity 



injection of hirudin into the animal. The diameter of the artery is 

 then measured. From these data the velocity of the blood flow can be 

 calculated by means of the formula 



volume (passing through the stromuhr) per second 

 sectional area of blood-vessels. 



If the capacity of the bulb up to the mark c is 5 c.c., and it was 

 filled six times in a minute, then the amount of blood passing through 

 the instrument would be 30 c.c. in one minute, or | c.c. in one second. 

 Supposing the diameter of the artery to be 2 mm., the sectional area is 

 Trr 2 and rate of flow can be calculated as follows : 



^ r , .. 

 Velocity = 



0'5 c.c. 500 c.mm. 



3-1416 x I 2 3-1416 



159 millimetres per second. 



Many other instruments have been devised, of which the most 

 useful is the photohsematachometer of Cybulski. This consists of two 

 vertical tubes united at the top, and opening 

 below into a horizontal tube, as shown diagram- 

 matically in fig. 6$. The proximal end of an 

 artery is attached to the instrument at A, the 

 blood escaping at B into the distal end of the 

 artery. The blood will rise higher in the tube C 

 than in the tube D, the difference in height of the 

 two .columns being directly proportional to the 

 velocity of the blood flow in the artery. A graphic 

 record is obtained by allowing a beam of light to 

 throw an image of the menisci of the columns of 

 fluid on to a moving photographic plate. To de- 

 termine the absolute velocity of the blood, the 

 instrument must be calibrated. It has the advan- 

 tage of giving not merely the average velocity of 

 the blood flow, but also the variations during ven- 

 tricular systole and diastole. In one experiment, 

 the velocity varied from 250 mm. per second during 

 systole to 127 mm. in diastole. 



THE CIRCULATION IN THE CAPILLARIES. 



On observing the flow of blood through the 

 small arteries and veins in the mesentery or web 

 of a frog, the red corpuscles are seen to occupy 

 the central part of the vessels (axial zone), and to 



be moving more rapidly than the peripheral layer of blood, in which are 

 found most of the leucocytes. The formation of the axial zone is due to the 



FIG. 62. Diagram 

 showing the prin- 

 ciple of Cybtilski's 

 photohfemataclio- 

 raeter. 



