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. Illustration: Fig. 12.—The Right femur sawed in two, lengthwise. (Showing arrangement of compact and cancellous tissue.) Experiment 5. Obtain a part of a beef shin bone, or a portion of a sheep’s or calf’s leg, including if convenient the knee joint. Have the bone sawed in two, lengthwise, keeping the marrow in place. Boil, scrape, and carefully clean one half. Note the compact and spongy parts, shaft, etc. Experiment 6. Trim off the flesh from the second half. Note the pinkish white appearance of the bone, the marrow, and the tiny specks of blood, etc. Knead a small piece of the marrow in the palm; note the oily appearance. Convert some marrow into a liquid by heating. Contrast this fresh bone with an old dry one, as found in the fields. Fresh bones should be kept in a cool place, carefully wrapped in a damp cloth, while waiting for class use.