One more thing you have to note. There is only one alimentary canal, one liver, one heart—but there are two kidneys and two lungs, the one on one side, the other on the other, and the one very much like the other. There are two arms and two legs, the one almost exactly like the other. There is only one head, but one side of the head is almost exactly like the other. One side of the vertebral column is exactly like the other—as are also the two halves of the brain and the two halves of the spinal cord.
In fact, if you were to cut your rabbit in half from his nose to his tail, you would find that except for his alimentary canal, his heart, and his liver, one half was almost exactly the counterpart of the other.
Such is the structure of a rabbit, and your own body, in all the points I have mentioned, is made up exactly in the same way.
WHAT TAKES PLACE WHEN WE MOVE. § III.
[13.] Let us now go back to the question. How is it that we can move about as we do? And first of all let us take one particular movement and see if we can understand that.
For instance, you can bend your arm. You know that when your arm is lying flat on the table, you can, if you like, bend the lower part of your arm (the fore-arm as it is called, reaching from the elbow to the hand) on the upper arm until your fingers touch your shoulder. How do you manage to do that?
Look at the bones of the arm in a skeleton. ([Frontispiece]; also [Fig. 3].) You will see that in the upper arm there is one rather large bone (H) reaching from the shoulder to the elbow, while in the fore-arm there are two, one (U) being wider and stouter than the other (Ra) at the elbow, but smaller and more slender at the wrist. The bone in the upper arm is called the humerus; the bone in the fore-arm, which is stoutest at the elbow, is called the ulna; the one which is stoutest at the wrist is called the radius. If you look carefully you will see that the end of the humerus at the elbow is curiously rounded, and the end of the ulna at the elbow curiously scooped out, in such a way that the one fits loosely into the other.
