296 PHYSIOLOGY. 



swiftness. If the time a corpuscle takes to traverse a course of a 

 known length be measured under the microscope, a fairly accurate 

 estimate can be made. Due allowances must be made for exaggera- 

 tions from the magnifying power of the microscope. 



It is thus estimated that the corpuscles traverse one inch per 

 minute through the capillaries in man. 



BLOOD=PRESSURE ARTERIAL, VENOUS AND CAPILLARY 



TENSION. 



The blood-pressure within any vessel may be looked upon as the 

 stress upon the inclosed liquid at the point of observation. Pressure 



Fig. 96. Showing the Relative Heights of Blood-pressure in Different 

 Blood-vessels. (YEO.) 



H, Heart. A, Arteries, a, Arterioles. c, Capillaries. V, Small veins. 



v, Large veins. H-7, Being the zero-line, the pressure is indicated by the 



elevations of the curve. The numbers on the left give the pressure in 

 millimeters of mercury. 



of the blood has been placed before the student's attention quite fre- 

 quently during the discussion of the circulation of this vital fluid. 

 Its consideration has been but superficial, however, up to this point. 



The blood-pressure depends upon three factors, the peripheral 

 resistance, the force of the heart and the quantity of the blood. 



If the quantity of the blood ejected by the heart decreases for a 

 time, other things being equal, the pressure falls. The acceleration of 



