[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.

A fresh or living bone is covered with a delicate, tough, fibrous membrane, called the periosteum.  It adheres very closely to the bone, and covers every part except at the joints and where it is protected with cartilage.  The periosteum is richly supplied with blood-vessels, and plays a chief part in the growth, formation, and repair of bone.  If a portion of the periosteum be detached by injury or disease, there is risk that a layer of the subjacent bone will lose its vitality and be cast off.[5]

30.  Microscopic Structure of Bone.  If a very thin slice of bone be cut from the compact tissue and examined under a microscope, numerous minute openings are seen.  Around these are arranged rings of bone, with little black bodies in them, from which radiate fine, dark lines.  These openings are sections of canals called Haversian canals, after Havers, an English physician, who first discovered them.  The black bodies are minute cavities called lacunae, while the fine lines are very minute canals, canaliculi, which connect the lacunae and the Haversian canals.  These Haversian canals are supplied with tiny blood-vessels, while the lacunae contain bone cells.  Very fine branches from these cells pass into the canaliculi.  The Haversian canals run lengthwise of the bone; hence if the bone be divided longitudinally these canals will be opened along their length (Fig. 13).

Thus bones are not dry, lifeless substances, but are the very type of activity and change.  In life they are richly supplied with blood from the nutrient artery and from the periosteum, by an endless network of nourishing canals throughout their whole structure.  Bone has, therefore, like all other living structures, a self-formative power, and draws from the blood the materials for its own nutrition.

[Illustration:  Fig. 13.