A STUDY OF THE CIRCULATION OF BLOOD 141 



is driven forward in jerks ; we, therefore, infer that we are 

 looking at arteries. As the arteries subdivide and become 

 smaller, the pulse gradually disappears, and in the capil- 

 laries there is a steady stream on toward the veins. Some 

 of the capillaries are so small that but a single corpuscle 

 can pass through at a time; 

 indeed, they are sometimes 

 squeezed out of shape within 

 the microscopic tube, but on 

 escaping into the larger blood 

 vessels they resume their 

 former shape (see Fig. 54). 



Absence of Pulse in Capillaries 

 and Veins. It is clear that if 

 the pulse were transmitted 

 from the arteries to the capil- 

 laries, the thin walls of the FIG. 54. Capillaries of Frog's Foot, 

 latter would be unable to with- The capillary tubes are more or less 

 stand the pressure; the blood filled with the oval red corpuscles, 

 would then escape from the The irregular black spots are the 



pigment cells which give color to 

 capillaries and flood the vari- the skin of the frog. 



ous tissues. Hence, there must 



be some means of reducing the pressure in the blood vessels 



before the capillaries are reached. 



At least two causes combine to produce the even flow of 

 blood in the capillaries and veins. In the first place, the 

 arteries near the heart expand at every contraction of the 

 ventricle, thus making room for the additional cupful of blood 

 that is forced out at each heart beat. This expansion of the 

 arteries, however, becomes less and less as the blood enters 

 the smaller branches. In the second place, if all the fine 

 capillaries of the body could be placed side by side, their 

 combined diameters would be many times the diameter of all 

 the arteries that supply them with blood. Hence, as the 

 blood is pushed outward from the heart, it finds more and 

 more room in which to flow. The size of each capillary is 



