Bones which, like the humerus and femur, present a shaft and articular extremities, are called long bones; other examples are tibia and fibula, radius and ulna, metacarpal and metatarsal bones, and the phalanges of fingers and toes. Tab-ular bones form thin plates, like those of the roof of the skull, and the shoulder-blades. Short bones are rounded or angular, and not much longer in one diameter than another ; as the carpal and tarsal (Fig. 19) bones. Irregular bones include all which do not fit well into any of the above classes; they usually lie in the middle line of the body and are divisible into similar right and left halves; the vertebræ are good examples.

What lies in the cavities of the spongy bone ?

Why are most bones hollow ? Which will bear most weight, a tube or a solid rod of the same material, weight, and length? Give illustrations. Why are grass stalks hollow ?

* Many of the bones of birds are thin-walled tubes of dense bone : the central cavity contains air and no marrow, and communicates by tubes with the lungs. Examine the humerus of a pigeon or a rooster.

All bones are covered by periosteum except where they enter into the formation of a joint, but in the human body only the long bones possess a medullary cavity containing yellow marrow. The rest are filled up by spongy bone, covered by a thin layer of dense, and have red marrow in their spaces.