A Practical Physiology | Page 9

not usually reckoned as separate bones, being a part of the structure of the skin.
The number of distinct bones varies at different periods of life. It is greater in childhood than in adults, for many bones which are then separate, to allow growth, afterwards become gradually united. In early adult life, for instance, the skull contains 22 naturally separate bones, but in infancy the number is much greater, and in old age far less.
The bones of the body thus arranged give firmness, strength, and protection to the soft tissues and vital organs, and also form levers for the muscles to act upon.
28. Chemical Composition of Bone. The bones, thus forming the framework of the body, are hard, tough, and elastic. They are twice as strong as oak; one cubic inch of compact bone will support a weight of 5000 pounds. Bone is composed of earthy or mineral matter (chiefly in the form of lime salts), and of animal matter (principally gelatine), in the proportion of two-thirds of the former to one-third of the latter.
[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.
[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