THE CIRCULATION OF THE BLOOD 147 



Experiment 42. Grasp the left wrist tightly with the thumb and 

 two fingers of* the right hand. Note the little knots or swellings in 

 the veins on the back of the hand, caused by checking the flow of the 

 venous blood towards the heart. These swellings show the position 

 of the valves of the veins. 



205. The Capillaries. Between the ends of the smallest 

 arteries and the beginnings of the tiniest veins is a very 

 close network, like the finest lace, with the minutest little 

 tubes for threads. These little tubes or threadlike vessels 

 are called capillaries. 



In reality the capillaries are as much smaller than threads 

 as the smallest threads are smaller than cables. So close 



FIG. 91. The Right Axillary 

 and Brachial Arteries, with 

 Some of their Main 

 Branches. 



set are these tiny vessels that we cannot prick any part of 

 the skin, even with the finest needle, without wounding 

 many of them and drawing blood. They are so small that 

 three thousand of them placed side by side would not, in 

 their united width, measure more than one inch. 



206. The Interchange between the Blood in the Capillaries 

 and the Lymph in the Tissues. The capillaries are closed 

 vessels. The space between the walls of the capillaries 



