THE CIRCULATION OF THE BLOOD AND LYMPH 119 



appearance of the pulse are the chief characteristics of 

 the capillary circulation. The explanation we have already 

 found in the great resistance of the narrow and much- 

 branched vessels. Although the average diameter of a 

 capillary is only about 10 //, (5 to 20 ^ in different parts of 

 the body), the number of branches is so prodigious that the 

 total cross-section of the systemic capillary tract has been 

 estimated at 500 to 700 times that of the aorta. 



The total cross-section of the vascular channel gradually 

 widens as it passes away from the left ventricle. In the 

 capillary region it undergoes a great and sudden increase. 

 At the venous end of this region the cross-section is again 

 somewhat abruptly contracted, and then gradually lessens as 

 the right side of the heart is approached ; but the united 

 sectional area of the large thoracic veins is greater than that 

 of the aorta. 



The blood-pressure in the capillaries has been measured 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 

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

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

 7 to 1 1 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 capil- 

 laries at the root of the nail in man varies from 30 to 50 mm. of 

 mercury. 



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 

 submaxillary gland are widened, and the vascular resistance 

 lessened, by the stimulation of the chorda tympani nerve, 

 the pulse passes through to the veins. And, normally, a 

 pulse may be seen in the wide capillaries of the nail-bed 

 especially 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 propor- 

 tional to the amount of resistance in the portion of the tube 



