THE MUSCULAR SYSTEM 



69 



between the two by the muscles which pass from the front 

 of the thigh to the hip bones. The raising of the hand by 

 bending the elbow joint is perhaps a clearer example of 

 the lever of the third class. The weight is at the end of 

 the forearm, the fulcrum is at the elbow, 

 and the power is between, at the point 

 on the radius where is inserted the muscle 

 which lies on the front of the humerus. 



88. Coordination of Muscular Action. 

 Our ordinary movements involve the use 

 of many different muscles, and very com- 

 plicated action of levers and cords. Even 

 simply to stand erect requires strong ten- 

 sion of certain muscles and ligaments 

 pulling against one another. The mus- 

 cles on the front of the thigh contract to 

 keep the knee from bending, while the 

 ligaments of the joint prevent it from 

 bending the wrong way. The muscles 

 on the front of the leg contract to keep 

 the body from falling backward, and those 

 at the back contract to keep it from fall- 

 ing forward. In the same way the trunk 

 is balanced on the thigh bones by the 

 muscles passing from the body to the 

 thigh in front and back, while a particu- Fig. 46. Diagram 

 larly strong ligament, crossing the hip us ' ^ 

 joint from the pelvis to the thigh bone, tend to keep the 

 keeps the extra backward weight of the body e 

 trunk from destroying the balance of the frame. At the 

 back of the neck are the muscles which give to the head 

 its erect and graceful poise, while many ligaments bind it 

 to the spinal column (Fig. 46). 



