If the humerus were divided lengthwise we would find that its shaft was hollow; the space is known as the medullary cavity, and in life is filled with soft fatty marrow. Fig. 22 represents such a longitudinal section. We see in it that the marrow cavity ends near the articular extremities; and that in these the bone has a loose, spongy texture, except a thin dense layer on the surface. In the shaft the compact outer layer is much the thicker, the spongy portion only forming a thin stratum next the medullary cavity.* To the unassisted eye the spongy bone appears made up of a trellis-work of thin bony plates which intersect in all directions and surround cavities about the size of the head of a small pin. In these spaces there is found during life a substance known as the red marrow, which is quite different from the yellow fatty marrow of the medullary cavity.*

Fig. 22. The humerus cut open, a, marrow cavity; b, hard bone; c, spongy bone; d, cartilage.

What do the knobs and rough patches on the bone indicate?

What should we find on dividing the humerus lengthwise? What is its shaft cavity called? What does it contain? Where does the marrow cavity end? What is the texture of the articular extremities of the bone? How does the shaft differ in structure from the extremities of the femur? What does the spongy bone look like?

* These facts may readily be demonstrated by sawing in two lengthwise the bones out of a leg of mutton.

Why Bones Are Hollow

If the bones were solid they would be extremely heavy and unnecessarily strong for the common purposes of life, unless only the same amount of material were used in their construction, and then they would be either loose in texture and easily broken, or, if dense, they would be thin rods and not give sufficient surface for the attachment of muscles! It is a well-known principle in practical mechanics that the same amount of material will bear a greater strain if in the form of a tube than in that of a solid rod of the same length; hence iron pillars are cast hollow; to fill them up solidly would make them enormously heavier without anything like a proportionate increase in strength. Take a glass tube one foot long and a piece of glass rod of the same length and weight; support each at its ends and hang weights on the middle until it breaks; the tube will be found to bear a very much greater strain before yielding. We see an application of this same method of utilizing a given amount of material to the best advantage for support, in the hollow stalks of grass, wheat, and barley.