In the leg below the knee, and in the forearm, we have two groups of "benders" or flexors, and "straighteners" or extensors, as in the upper arm and leg, only slenderer and more numerous. They taper down into cord-like tendons at the wrist and ankle to fasten and to pull the hands and feet "open" and "shut," just as do the strings in the legs and arms of a puppet or mechanical doll, or the sinews in the foot of a chicken.
You can easily prove that a bone is made up of living tissue soaked and stiffened with lime, by putting it into a jar filled with weak acid. This will gradually dissolve and melt out the lime salts, and then you will find that the bone has lost three-fourths of its weight and that what remains of it is so soft and flexible that it can be bent, or even tied into a knot.
The hollow spaces in the bones of birds, however, are filled with air, which makes them lighter for flying.
To give you an idea of what real things nerve-trunks are, this sciatic nerve is as large as a small clothes-line, or, more accurately, as a carpenter's lead pencil, and so strong that when the surgeon cuts down upon it and stretches it to cure a very bad case of sciatica, he can lift the lower half of the body clear of the table by it. This strength, of course, is not due to the nerve-fibres and cells themselves but to the tough, fibrous sheath, or covering, with which all the nerves that run outside of the brain and spinal cord are covered and coated. The spinal cord, though it is between one-half and three-fourths of an inch across, or about the size of an ordinary blackboard pointer, has little or none of this fibrous tissue in it, and is very soft and delicate, easily torn when its bony case is broken; hence its old name, the spinal marrow, from its apparent resemblance to the marrow, or soft fat, in the hollow of a bone.
Some of these coal-tar remedies are Acetanilid, and Antipyrin, and Phenacetin.