=Gross Structure of Bones.=--The structure of a long bone is shown in Fig. 29. It has a long, _hollow shaft_ of hard, compact bone, and _enlarged ends_ composed of spongy bone. The hollow in the shaft is _filled with yellow marrow_, which is composed of blood vessels and fat, and aids in nourishing the bone. The long bones are found in the limbs (Fig. 28). The ribs and other flat bones and the irregular bones contain no yellow marrow; they are spongy inside, and hard and compact near the surface. There is a _red marrow_ in the cavities in the spongy parts of bones (Fig. 29). _New red blood cells are formed in this marrow._ The bones have a close-clinging, fibrous covering composed of connective tissue and blood vessels. It is called _periosteum_. [Illustration: FIG. 29.--FEMUR, sawed lengthwise. The red blood cells are formed in the red marrow of the spongy part.] [Illustration: FIG. 30.--FRONT VIEW OF RIGHT FEMUR.] =Chemical Composition of Bone.=--Experiments (2 and 3) show that the bones contain a _mineral or earthy substance_, which makes them hard and stiff, and a certain amount of _animal matter, called gelatine_, which binds the mineral matter together and makes the bones tough and somewhat elastic. The fire burned out the animal matter of the first bone, and the acid dissolved out the mineral matter of the second bone. _The mineral matter is chiefly lime, and makes up about two thirds of the weight of the bone._ (Why is more mineral than animal matter needed?) The animal gelatine is a gristly substance. As the body grows old, the animal matter of the bones decreases, and they become lighter. They are more easily broken and do not heal so readily as the bones of young persons. =The skeleton is subdivided= into the bones of the _head, trunk, and limbs_. The bones of the trunk are those of the spine, the chest, the shoulder blades, collar bone, and hip bones. [Illustration: FIG. 31.--VERTEBRAL COLUMN. Side view.] =The spinal or vertebral column= is made up of twenty-six bones (Fig. 31). It is the axis of the human skeleton, to which all other bones are directly or indirectly attached. Animals with inside skeletons have this column, and are called vertebrates. Fish, reptiles, birds, beasts, apes, and man are vertebrates. The spine, as this column is sometimes called, is not only the main connecting structure and support of the body, but it forms a channel through which passes the spinal cord.