Page:Popular Science Monthly Volume 19.djvu/670

652 the skin. This is the physiology of blushing. Congestion is produced by a permanent expansion of the capillaries. We might call blushing a momentary congestion.

In an emergency the arteries are capable of great expansion. They are connected by branches, or loops; and, in case of stoppage of the circulation in a large artery, either by disease or a surgical operation, another will, after a time, perhaps a few hours, expand to a size requisite to carry sufficient blood. This variation in the carrying capacity of the arteries is the important secondary means of adapting the amount of blood to the wants of any part of the body.

In man there is a greater and more direct supply of blood to the right arm, corresponding to the greater use of that limb. But, in birds, equality of supply is necessary for the equality of strength needed in steady flight.

For protection, the arteries are as deep-seated as possible, lying beneath the muscles, and appearing rarely at the surface. At the joints they form loops, so that the circulation may not be stopped by compression of a single trunk. A fine example of adaptation is seen in the arm of the lion, where the main artery, to be protected by the powerful muscles, passes through a perforation in the bone.

 

ew realize the great practical importance of extreme accuracy in standards of weight and extension, and it is not generally known what degree of accuracy has been attained in the measurement of the standards of length now in use in different countries. The carpenter's foot-rule and the tailor's yard are familiar articles, but, if asked, probably neither the carpenter nor the tailor could tell whether there is any means by which the true length of a foot or a yard can be determined. It is clear, however, that there must be a standard with which the common measures should be made to agree, in order to have the same absolute value. But we may reflect that the constant use of any measure will change its length, and that it will eventually become worn out. We can, then, readily understand the great value of an accepted standard, from which copies can be made, thus preventing any gradual alteration in our measures. Such standards of reference are properly held in the custody of national governments, scientific societies, and institutions.

It is by no means a simple process to compare one measure with another, and to determine the variation between the two. On the contrary, the utmost skill and long experience are required for such 