120 



A MANUAL OF PHYSIOLOGY 



heart is approached ; but the united sectional area of the large 

 thoracic veins is greater than that of the aorta. 



Attempts have been made to measure the blood-pressure in the 

 capillaries by weighting a small plate of glass laid on the back of one 

 of the fingers behind the nail, until the capillaries are just emptied, 

 as shown by the paling of the skin (v. Kries), or by observing the 

 height of a column of liquid that just stops the circulation in a trans- 

 parent part (Roy and Graham Brown). The last-named observers 

 found that a pressure of 100 to 150 mm. of water (about 7 to n mm. 

 of Hg) was needed to bring the blood to a standstill in the capillaries 

 and veins of the frog's web ; that is, about a third of the blood- 

 pressure in the frog's aorta. The pressure in the capillaries at the 

 root of the nail in man varies from 30 to 50 mm. of mercury, as 

 estimated by the method of v. Kries. But the method is exposed 

 to serious errors. 



Under certain conditions the pulse-wave may pass into the 

 capillaries and appear beyond them as a venous pulse. Thus, 



we shall see that 

 when the small 

 arteries of the sub- 

 maxillary gland 

 are widened, and 

 the vascular re- 

 sistance lessened, 

 by the stimulation 

 of the chorda tym- 

 pani nerve, the 

 pulse passes 

 through to the 



FIG. 50. RELATION OF BLOOD-PRESSURE, VELOCITY, 

 AND CROSS-SECTION. 



The curves P, V, and S represent the blood-pressure, 

 velocity of the blood, and total cross-section respectively 

 in the arteries A, capillaries C, and veins V. 



veins. And, nor- 

 mally, a pulse may 

 be seen in the wide 

 capillaries of the 

 nail - bed especi- 

 ally when they are partially emptied by pressure as a flicker 

 of pink that comes and goes with every beat of the heart. 



We have seen that the lateral pressure at any point of a uniform 

 rigid tube through which water is flowing is proportional to the 

 amount of resistance in the portion of the tube between this point 

 and the outlet. In any system of tubes the sum of the potential and 

 kinetic energy must diminish in the direction of the flow ; and 

 although the problem is complicated in the vascular system by the 

 branching of the channel and the variation in the total cross-section, 

 yet theory and experiment agree that in the larger arteries the 

 lateral pressure diminishes but slowly from the heart to the periphery, 

 the resistance being small compared with the resistance of the whole 

 circuit. In the capillary region the vascular resistance abruptly 

 increases ; the velocity (and therefore the kinetic energy) abruptly 

 diminishes, and the lateral pressure falls much more steeply between 

 the beginning and the end of this region than between the heart and 



