5. THE STRUCTURE OF THE BONES.--If we examine one of the long bones, which has been sawn through lengthwise, we observe that it is admirably fashioned for affording lightness as well as strength (Fig. 1). Its exterior is hard and resisting, but it is porous at the broad extremities, while through the central portion there is a cavity or canal which contains an oily substance, called _marrow_. Let us now take a thin section of bone, and examine it under the microscope; we discover that it is pierced by numerous fine tubes (Fig. 2), about which layers of bone-substance are arranged. Accordingly, though a bone be as hard as stone externally, it is by no means as heavy as stone, by reason of its light interior texture. Another element of power is found in the curved outline of the bones. The curved line is said to be "the line of beauty," as it certainly is the line of strength, and is uniformly employed in the bones whose position exposes them to accident. {18} [Illustration: FIG. 3.--THE SKELETON] {19} [Sidenote: 6. Number of bones? Skeleton? The skull? Chest? The trunk? The trunk and skull, how maintained? What of the arms? Legs?] 6. THE SKELETON.--The number of bones in the human body exceeds two hundred. When these are joined together in the proper places, they form what is termed the _Skeleton_ (Fig. 3). It embraces three important cavities. The first, surmounting the frame, is a box of bone, called the _skull;_ below this, is a bony case, or "chest;" and lower down is a bony basin, called the _pelvis_. The two latter compose the trunk. The trunk and skull are maintained in their proper relations by the "spinal column." Branching from the trunk are two sets of limbs: the arms, which are attached to the chest by means of the "collar-bone" and "shoulder-blade;" and the legs, directly joined to the lower part of the trunk. [Sidenote: 7. Design of the cavities? Give the examples.] 7. The cavities of which we have spoken, are designed for the lodgment and protection of the more delicate and perishable parts of the system. Thus, the skull, together with the bones of the face, shelters the brain and the organs of four senses--sight, hearing, smell, and taste. The chest contains the heart, lungs, and great blood-vessels, while the lower part of the trunk sustains the liver, stomach, and other organs. [Sidenote: 8. Joint or articulation? Movable joints, how compacted? The ligaments of the movable joints? What is a sprain? Consequence of a serious sprain?]