The Gross Structure Of Bones

Although the bones differ very much in shape all are alike in microscopic structure and in chemical composition. When alive they have a bluish-white color, with a pinkish hue when blood is flowing through them; they possess considerable flexibility and elasticity, which may be best observed in a long slender bone, as a rib.*

To get a general idea of the structure of a bone we may select the humerus (Fig. 21). When fresh this is Closely invested on its outside by a tough membrane, the periosteum, composed of connective tissue and containing many blood-vessels. On its under side new bony tissue is deposited as long as the bone is growing thicker, and throughout life it is concerned in the nourishment of the bone, which dies if it be stripped off.* The periosteum covers the humerus except on its ends (Cp, Tr, Cpl) at the shoulder and elbow-joints; there the bone is covered by a thin layer of gristle or cartilage. Very early in life the whole humerus consists of cartilage; this is afterwards absorbed and replaced by bone, leaving only a thin layer of articular cartilage on each end.

How do bones differ from one another? In what respects do all bones agree? What is the color of a living bone? Name some mechanical properties of bone. In what bones may such properties be most readily seen?

What covers a bone on the exterior? What is it composed of? Does it contain bloodvessels?

* The rib of a sheep or a rabbit when thoroughly boiled can be readily scraped clean and preserved, and serves admirably to show the flexibility and elasticity of bearing surface in the joints, and also to provide space on which to attach the muscles which move the bone ; the various knobs on the extremities, and the rough patches on the shaft, all mark areas where muscles were fixed.

Fig. 21. The right humerus, seen from the front For description, see text.

The bone itself consists of a central nearly cylindrical portion or shaft (extending between the dotted lines X and Z) and two articular extremities. These extremities are enlarged to give a wider.

What are the functions of the periosteum? Where is the periosteum absent? Of what does the humerus consist in very early in life? What happens to most of its cartilage afterwards? Where is some cartilage left?

What are the main divisions of the humerus? What is the general form of its shaft? Why are its articular extremities large?

* Cases have been recorded in which a considerable portion of a bone or even the whole bone has been removed during life, and the periosteum (left but slightly injured) has formed a new bone in place of the old.