110 PHYSIOLOGY 



Relation of Muscles to Bones. — Bones have no power 

 of themselves to move. Their movement is entirely due 

 to the contraction of muscles that are attached to them 

 (Fig. 66). An infinite variety of movement is possible 

 through the co-ordinated activity of a great number of 

 muscles. These vary greatly in shape and size and 

 are arranged about the skeleton in such a way that 

 the beautiful symmetry of the body is maintained and 

 the greatest amount of work accomplished with the least 

 expenditure of energy. 



If we feel the muscles through the skin when they 

 contract we may notice that the active muscle is fre- 

 quently situated some distance from the point of motion. 

 The muscles that move the upper arm are situated on the 

 trunk, those that move the lower arm are situated on the 

 upper arm and those that move the fingers are situated 

 close to the elbow. The weight of the muscle thus rests 

 on the stationary bone and the other bone is free to 

 move. The muscles are fastened to the bones by inelas- 

 tic tendons that are sometimes very long. The tendons 

 economize space and slip easily over the moving joint. 



The muscles are arranged about a joint so that its 

 motion is perfectly controlled ; the number depends upon 

 the character of the joint and the character of its work. 

 In the shoulder where a ball and socket permit rotary 

 motion there are a large number of muscles, but in 

 hinge joints which merely permit flexion and extension, 

 only two controlling muscles are necessary though in 

 powerful action these may be reinforced by others. 



Movement is always due to contraction, not to relaxa- 

 tion. Usually muscles are arranged in antagonistic pairs 



