934 



PHYSIOLOGY 



tubes are united at their upper extremities. In this case so long as the blood flows 

 from a to 6, the fluid in h will rise higher than in h', and the difference in height of the 

 fluid in the two tubes will be proportional to the velocity of the blood. A graphic record 

 of this difference of pressure is obtained by allowing a narrow beam of light to throw an 

 image of the menisci of the two columns of fluid through a slit on to a moving photo- 

 graphic plate. Such a record is given in Fig. 401. The width of the black space at any 

 point is proportional to the velocity of the blood at the moment at which this part of 



Fia. 401. Record of blood velocity in the carotid artery of the rabbit. (CYBTJLSKI.) 



the record was being taken. Of course this instrument has to be calibrated if we wish to 

 determine the velocity of the blood in absolute measure. In Fig. 401 the velocity at the 

 points 1 and 1', corresponding to the cardiac systole, was 248 mm. per second. At 2 and 

 2', corresponding to the dicrotic elevation, the velocity was also 248 mm. At 3 and 3', 

 towards the end of diastole, the velocity sank to 127 mm. 



The velocity of the blood in the capillaries can be measured by direct observation 

 of the capillaries under the microscope, and noting the time it takes for a blood corpuscle 

 to move from one edge of the field to the other. 



THE VELOCITY IN DIFFERENT PARTS OF THE 

 VASCULAR SYSTEM 



During systole the velocity of the blood in any part of the arterial system 

 must be greater than during diastole ; thus in the carotid of the horse the 

 following figures were found : 



During systole 

 During diastole 



Velocity per second 

 520 mm. 

 150 mm. 



The following figures of the average velocity have been obtained from 

 experiments oh dogs (Tigerstedt) : 



