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