Page:Popular Science Monthly Volume 27.djvu/814

792 the wonderful power which we so often see in growth, in repairing damages, and in providing for new conditions in cases of accidents. This inclusion of the astragalus in the tibia does not occur in the reptiles, but appears first in the mammalia, which descended from them.

The same active cause that produced the two grooves of the lower end of the leg produced the groove of the middle of the upper end of the astragalus. Here we have the yielding lower end of the tibia resting on the equally spongy material of the middle of the astragalus. There is here no question of the hard material cutting into soft, but simply the result of continuous concussion. The consequence of concussion would be to cause the yielding faces of the bones to bend downward in the direction of gravity. If they were flat at first they would begin to hollow downward, and a tongue above and groove below would be the result. And that is exactly what has happened. Without exception, every line of mammalia commenced with types with an astragalus which is flat in the transverse direction, or without median groove. From early tertiary times to the present day, we can trace the gradual development of this groove in all the lines which have acquired it. The upper surface becomes first a little concave; the concavity gradually becomes deeper, and finally forms a well marked groove.

The history of the wrist-joint is similar. The surface of the forearm bones which joins the fore-foot is in the early tertiary mammalia uniformly concave. In the ruminating mammals it is divided into three fossæ, which are separated by sharp keels. These fossæ correspond with the three bones which form the first row of the carpus or palm. The keels correspond to the sutures between them. The process has been evidently similar to that which has been described above as producing the side-grooves in the end of the tibia. The dense walls of the sides of the three bones impinging endwise on the broad yielding surface of the fore-arm (radius) have gradually, under the influence of countless blows, impressed themselves into the latter. On the contrary, the surface above the weaker lines between the bones not having been subject to the impact of the blows, and influenced by gravity, remains to fill the grooves, and to form the keels which we observe.

There is another striking instance of the same kind in the feet of mammalia; that is, in the development of the keels and grooves which appear at the articulation of the first set of bones of the toes (metapodials) with the bones of the second set (phalanges). These keels first appear on the posterior side of the end of the first set of bones, projecting from between two tendons. These tendons, in many mammals, contain two small bones, one on each side, which act like the knee-pan, and resemble it in miniature, which are called sesamoid bones. These tendons and bones exercise a constant pressure on each side of the middle line, when the animal is running or walking, and