79 2 THE POPULAR SCIENCE MONTHLY. 



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 rep- 

 tiles, 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 con- 

 cussion 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 fore- 

 arm bones which joins the fore-foot is in the early tertiary mammalia 

 uniformly concave. In the ruminating mammals it is divided into 

 three fossae, which are separated by sharp keels. These fossae cor- 

 respond 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 (meta- 

 podials) 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 



