This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
The blood leaves the heart intermittently and not in a regular stream, a quantity being forced out at each systole of the ventricles; before it reaches the capillaries, however, this rhythmic movement is transformed into a steady flow, as may readily be seen by examining with a microscope thin transparent parts of various animals, as the web of a frog's foot, a bat's wing, or the tail of a small fish. In consequence of the steadiness with which the capillaries supply the veins the flow in these latter is also unaffected directly by each beat of the heart; if a vein be cut the blood wells out uniformly, while a cut artery spurts out with much more force, and in jets which are more powerful at regular intervals corresponding with the contractions of the ventricles.
There is no more fascinating or instructive spectacle than the circulation of the blood as seen with the microscope in the thin membrane between the toes of a frog's hind limb. Upon focusing beneath the outer layer of the skin a network of minute arteries, veins, and capillaries, with the blood flowing through them, comes into view (Fig. 61). The arteries, a, are readily recognized by the fact that the flow in them is fastest and from larger to smaller branches. The smallest are seen to end in capillaries, which form networks, the channels of which are all nearly equal in size. In the veins arising from the capillaries the flow is from smaller to larger trunks, and slower than in the arteries, but faster than in the capillaries.
How by exercise? Why should an invalid's pulse not be felt when he is excited? How does age affect the pulse rate?
In what manner does the blood leave the heart? How is its flow altered before reaching the capillaries? How may this be observed? Is the flow in the veins rhythmic or steady? Why? How does the bleeding from a cut artery differ from that of a cut vein?
What comes into view on examining a frog's web with a microscope?