ABSENCE OF PULSE Iff THE CAPILLARIES . 227 



both while the heart is contracting and between two heart- 

 beats. The heart, in fact, keeps the big elastic arteries 

 over-distended with blood; before they have had time to 

 nearly empty, another systole occurs and fills them up tight 

 again; so all the while the walls of the arteries are stretched 

 and keep pressing on the blood inside them, and steadily 

 forcing it on into the capillaries. The heart keeps the 

 arteries over-full, and the stretched elastic arteries drive the 

 blood through the capillaries. As the arteries are always 

 stretched and always pressing on the blood the capillaries 

 receive a steady supply, and the flow through them is uni- 

 form. This even capillary flow passes on a steady blood 

 stream to the veins.* 



The object of having no pulse in the capillaries is to 

 diminish the danger of their rupture. As we have seen, 

 materials from the blood have to ooze through their walls 

 to nourish the organs of the body, and wastes from the 

 organs to soak back into the blood that they may be carried 

 off. Their walls have therefore to be very thin; and if the 



When? What does the heart do? What happens before the 

 arteries have had time to empty? What is the condition of the 

 arterial walls all through life? What results from their stretched 

 condition? What keeps the arteries tightly filled? What sends 

 blood through the capillaries? How do the capillaries get a steady 

 blood supply? Why do we find a uniform current in the veins? 



What is gained by having no pulse in the capillaries? What 

 must food materials in the blood do before they can nourish the 

 body? What must the wastes of the organs do? Why must the 

 capillary walls be very thin? 



* " Every inch of the arterial system may, in fact, be considered as convert- 

 ing a small fraction of the heart's jerk into a steady pressure, and when all these 

 fractions are summed up together in the total length of the arterial system no 

 trace of the jerk is left. As the effect of each systole becomes diminished in 

 the smaller vessels by the causes above mentioned, that of this constant pres- 

 sure becomes more obvious, and gives rise to a steady passage of the fluid from 

 the arteries towards the veins. In this way, in fact, the arteries perform the 

 same functions as the air-reservoir of a fire-engine, which converts the jerk- 

 ing impulse given by the pumps into the steady flow of the delivery hose."- 

 HUXLEY. 



