[Illustration: Skeleton of a man. _CR._, cranium; _CL._, clavicle; _ST._, sternum; _H._, humerus; _V.C._, vertebral column; _R._, radius; _U._, ulna; _P._, pelvic girdle; _C._, carpals; _M._, metacarpals; _Ph._, phalanges; _F._, femur; _Fi._, fibula; _T._, tibia; _Tar._, tarsals; _MT._, metatarsals.] Bones and Muscles.--The body is built around a framework of bones. These bones, which are bound together by tough _ligaments_, fall naturally into two great groups, the bones of the body proper, vertebral column, ribs, breast bone, and skull, which form the _axial_ skeleton, and the appendages, two sets of bones which form the framework of the arms and legs, which with the bones which attach them to the axial skeleton form the _appendicular_ skeleton. To the bones are attached the muscles of the body. Movement is accomplished by contraction of muscles, which are attached so as to cause the bones to act as levers. Bones also protect the nervous system and other delicate organs. They also help to give form and rigidity to the body. [Illustration: Diagram showing action of biceps muscle. _a_, contracted; _b_, extended; _h_, humerus; _s_, scapula.] Hygiene of Muscles and Bones.--Young people especially need to know how to prevent certain defects which are largely the result of bad habits of posture. Standing erect is an example of a good habit, round shoulders a bad habit of this sort. The habit of a wrong position of bones and muscles once formed is very hard to correct. This can best be done by certain corrective exercises at home or in the gymnasium. Round shoulders is most common among people whose occupation causes them to stoop. Drawing, writing, and a wrong position when at one's desk are among the causes. Exercises which strengthen the back muscles and cause the head to be kept erect are helpful in forming the habit of erect carriage. Slight curvature of the spine either backward or forward is helped most by exercises which tend to straighten the body, such as stretching up with the hands above the head. Lateral curvature of the spine, too often caused by a "hunched-up" position at the school desk, may also be corrected by exercises which tend to lengthen the spinal column. [Illustration: Three classes of levers in the human body; bones and muscles act together. _A_, a lever of the first class; _B_, a lever of the second class; _C_, a lever of the third class.] It is the duty of every girl and boy to have good posture and erect carriage, not only because of the better state of health which comes with it, but also because one's self-respect demands that each one of us makes the best of the gifts that nature has given us. An erect head, straight shoulders, and elastic carriage go far toward making their owner both liked and respected. [Illustration: Bad posture in the schoolroom may cause permanent injury to the spine.]