174 THE CIRCULATION 



ing tens, or even hundreds of these vessels. (Head Notes 12 

 and 13.) 



45. The capillaries are elastic, and may so expand as to pro- 

 duce an effect visible to the naked eye. If a grain of sand, or 

 some other foreign particle, lodge in the eye, it will become 

 irritated, and in a short time the white of the eye will be 

 " blood-shot." This appearance is due to an increase in the 

 size of these vessels. A blush is another example of this, but 

 the excitement comes throiigh the nervous system, and the 

 cause is some transient emotion, either of pleasure or pain. 

 Another example is sometimes seen in purplish faces of men 



12. Course of the Blood in the Capillaries. "The phenomena of 

 the capillary circulation are only observable with the aid of the micro- 

 scope. It was not granted to the discoverer of the circulation to see the 

 blood moving through the capillaries, and he never knew the exact mode 

 of communication between the arteries and veins. After it was pretty 

 generally acknowledged that the blood did pass from the arteries to the 

 veins, it was disputed whether it passed in an intermediate system of ves- 

 sels, or became diffused in the substance of the tissues, like a river flow- 

 ing between numberless little islands, to be collected by the venous 

 radicles and conveyed to the heart. Accurate microscopic investigations 

 have now demonstrated the existence, and given us a clear idea of the 

 anatomy, of the intermediate vessels. In 1661 the celebrated anatomist 

 Malpighi first saw the movement of the blood in the capillaries, in the 

 lungs of a frog. This spectacle has ever since been the delight of the 

 physiologist. We see the great arterial rivers, in which the blood flows 

 with wonderful rapidity, branching and subdividing, until the blood is 

 brought to the superb network of fine capillaries, where the corpuscles 

 dart along one by one, the fluid then being collected by the veins, and 

 carried in great currents to the heart." Flint. 



13. Experiment. Circulation. The circulation of the blood can be 

 fairly well illustrated by the aid of a "Davidson " syringe, some rubber 

 tubing, and a piece of glass tubing. 



The bulb of the syringe represents the heart, the elastic tubing the 

 arteries, and the fine glass tubing the capillaries. 



Attach the rubber tubing to the smallest nozzle of the syringe (the 

 tubing should be highly elastic, the black kind is the best ; it should be 

 about one-fourth inch diameter), place the other end of the syringe in a 

 vessel filled with water. 



Upon compressing the bulb of the syringe, the water will be seen to 

 issue from the tubing in jets corresponding with the compressions of the 

 bulb, but a little retarded ; placing the finger on the tubing but little 



45. Elasticity of the capillaries ? Grain of sand in the eye ? Blush ? Other cases ? 



