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.

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