324' MAN, A MAMMAL 



five hundred muscles, varying from one smaller than a pinhead to 

 a band almost two feet in length. Every movement of the body, 

 be it merely a change of expression or change in the pitch of the 

 voice, directly results from contraction of a muscle. Muscles also 

 give form to the body, and are useful in protecting the delicate 

 organs ^nd large blood vessels within them. 



Muscles and the Skeleton. — Muscles would be of little use to 

 animals if they were not attached to hard parts of the body which 

 serve as levers. In many invertebrate animals (for example, 

 crustaceans, insects, and moUusks), the muscles are attached to the 

 exoskeleton. In man they are attached to the endoskeleton. 



In the hind leg of the frog, if we cut tlirough the muscles of the thigh 

 to the bone, we may make out exactly how and where the muscles of 

 the thigh are attached to the bone. Moving the leg 

 in as many different directions as possible, we notice 

 that it may be flexed or bent ; that it may be ex- 

 tended to its original position ; that it may be moved 

 to and from the midline of the body ; that, with the 

 knee held stiff, the whole limb may be made to de- 

 scribe the arc of a circle.' 



These same movements are possible in the leg of 



a man. This movement between bones is obtained 



by means of joints. If, in the frog, we carefully 



separate the muscles of the thigh to the bone, we 



find that they are attached to the bone by white, 



glistening tendons. Careful examination shows that 



the bones themselves are held together by very tough 



Hinge joint, showing ^T^ita bands or cords ; these are the ligaments. We 



+°"h'^ ^ rw ^^ ' ^ iaiA, too, that one end of the large thigh bone fits 



into a socket in the hip bone or pelvic arch. It is 



thus easy to see how such free movement is obtained in the leg. 



Levers in the Body. — It is evident that movement of a joint is caused 

 by muscles which act in cooperation ^ith the bones to which they are at- 

 tached ; the latter thus form true levers. A lever is a structure by which 

 either greater work "power or greater range oj motion is obtained. In 

 this apparatus, the le\'er works against a fixed point, the fulcrum, in order 

 to raise a certain weight. A seesaw is a lever ; here the fulcrum is in the 

 middle, the weight is at one end, and the power to lift the weight is ap- 

 phed at the other end. There are three classes of levers, named accord- 

 ing to the position of the fulcrum. 



In the first class, the fulcrum lies between the weight and the power; 



' At this point demonatration with a human skeleton should be made. 



