. Physiology, experimental and descriptive . .Fig. 27. Cross Section of Small Arteryand Vein. The blood cannot do its work till it gets into the tissues, andto reach the tissues it must soak through the walls of thecapillaries. The work of the heart and arteries is to keep a steady flow of blood throughthe capillaries, that the tis-sues may be constantly sup-plied. How is it that the jerkyaction of the heart, at eachcontraction sending a jet ofblood into the arteries, —shown by a spurt when anartery is severed, and alsoindicated by the intermittentpulse,«— how is this inter-mittent flow conver
. Physiology, experimental and descriptive . .Fig. 27. Cross Section of Small Arteryand Vein. The blood cannot do its work till it gets into the tissues, andto reach the tissues it must soak through the walls of thecapillaries. The work of the heart and arteries is to keep a steady flow of blood throughthe capillaries, that the tis-sues may be constantly sup-plied. How is it that the jerkyaction of the heart, at eachcontraction sending a jet ofblood into the arteries, —shown by a spurt when anartery is severed, and alsoindicated by the intermittentpulse,«— how is this inter-mittent flow converted into the steady, uniform current thatwe have seen in the capil-laries ? A few experiments maymake this matter more : — 1. A common rubber syr-inge. 2. A glass tube threefeet long and seven-six-teenths of an inch outsidediameter. 3. Four inches of thesame size glass tubing, formaking connections. 4. Several nozzles, made of the same size glass tubing, all fine, but of varying degreesof fineness. Surface View. Longitudinal Section Fig. 28. Capillaries, composed of a single layer of cells. 80 EXPERIMENTS WITH BLOOD-FLOW. 5. India-rubber tubing, twelve feet, three-eighths of aninch inside diameter. This should be black, pure gum, rubberwhich is more highly elastic than the other kinds. 6. Three feet of rubber tubing, same size as above. 7. Four inches of white rubber tubing, same size as above,for making connections. In all the experiments, have one of the students assist byholding the outlet tube, so that (1) all the members of theclass may see the stream, and (2) that the stream may besuitably directed, as into a pail or sink. Count aloud, to mark the exact time of each compressionof the bulb, so the students can compare this with the timeand duration of the jets of water. Be very careful to use perfectly clean water, as any fineparticles of sediment drawn into the tube are likely to clogthe fine outlet of the nozzle. And it is well to take thefurther pre
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