[Illustration:  Fig. 10.—­The Skeleton.]

The proportion of earthy to animal matter varies with age.  In infancy the bones are composed almost wholly of animal matter.  Hence, an infant’s bones are rarely broken, but its legs may soon become misshapen if walking is allowed too early.  In childhood, the bones still contain a larger percentage of animal matter than in more advanced life, and are therefore more liable to bend than to break; while in old age, they contain a greater percentage of mineral matter, and are brittle and easily broken.

Experiment 3. To show the mineral matter in bone.  Weigh a large soup bone; put it on a hot, clear fire until it is at a red heat.  At first it becomes black from the carbon of its organic matter, but at last it turns white.  Let it cool and weigh again.  The animal matter has been burnt out, leaving the mineral or earthy part, a white, brittle substance of exactly the same shape, but weighing only about two-thirds as much as the bone originally weighed.

Experiment 4. To show the animal matter in bone.  Add a teaspoonful of muriatic acid to a pint of water, and place the mixture in a shallow earthen dish.  Scrape and clean a chicken’s leg bone, part of a sheep’s rib, or any other small, thin bone.  Soak the bone in the acid mixture for a few days.  The earthy or mineral matter is slowly dissolved, and the bone, although retaining its original form, loses its rigidity, and becomes pliable, and so soft as to be readily cut.  If the experiment be carefully performed, a long, thin bone may even be tied into a knot.

  [Illustration:  Fig. 11.—­The fibula tied into a knot, after the hard
  mineral matter has been dissolved by acid.]

29.  Physical Properties of Bone.  If we take a leg bone of a sheep, or a large end of beef shin bone, and saw it lengthwise in halves, we see two distinct structures.  There is a hard and compact tissue, like ivory, forming the outside shell, and a spongy tissue inside having the appearance of a beautiful lattice work.  Hence this is called cancellous tissue, and the gradual transition from one to the other is apparent.

It will also be seen that the shaft is a hollow cylinder, formed of compact tissue, enclosing a cavity called the medullary canal, which is filled with a pulpy, yellow fat called marrow.  The marrow is richly supplied with blood-vessels, which enter the cavity through small openings in the compact tissue.  In fact, all over the surface of bone are minute canals leading into the substance.  One of these, especially constant and large in many bones, is called the nutrient foramen, and transmits an artery to nourish the bone.

At the ends of a long bone, where it expands, there is no medullary canal, and the bony tissue is spongy, with only a thin layer of dense bone around it.  In flat bones we find two layers or plates of compact tissue at the surface, and a spongy tissue between.  Short and irregular bones have no medullary canal, only a thin shell of dense bone filled with cancellous tissue.