40. The circulation of the blood, as seen under the microscope, in the transparent web of a frog's foot, is a spectacle of rare beauty, possessing more than ordinary interest, when we consider that something very similar is taking place in our own bodies, on a most magnificent scale. It is like opening a secret page in the history of our own frames. We there see distinctly the three classes of vessels with their moving contents; first, the artery, {119} with its torrent of blood rushing down from the heart, secondly, the vein, with its slow, steady stream flowing in the opposite direction; and between them lies the network of capillaries, so fine that the corpuscles can only pass through "in single file." The current has here an uncertain or swaying motion, hurrying first in one direction, then hesitating, and then turning back in the opposite direction, and sometimes the capillaries contract so as to be entirely empty. Certain of the tissues are destitute of capillaries; such are cartilage, hair, and a few others on the exterior of the body. In all other structures, networks of these vessels are spread out in countless numbers: so abundant is the supply, that it is almost impossible to puncture any part with the point of a needle without lacerating tens, or even hundreds of these vessels. [Illustration: FIG. 32.--WEB OF A FROG'S FOOT, slightly magnified.] [Illustration: FIG. 33.--MARGIN OF FROG'S WEB magnified 30 diameters.] [Sidenote: 41. Elasticity of the capillaries? Grain of sand in the eye? Blush? Other cases?] 41. The capillaries are elastic, and may so expand as to produce 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 {120} increase in the size of these vessels. A blush is another example of this, but the excitement comes through 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 addicted to drinking brandy; in them the condition is a congestion of the capillary circulation, and is permanent, the vessels having lost their power of elastic contraction. [Sidenote: 42. Show what time is required for a given portion of blood to travel once around the body.]