MUSCLES AND TENDONS 345 



the muscle itself grows shorter. At a movable joint two 

 muscles usually act in opposition to each other, like two 

 springs on opposite sides of a door. If one muscle 

 shortens itself, the other grows longer. The two give a 

 forward and backward motion to the bone, like that 

 which we see when we raise and lower the forearm. 



Tendons are the extensions of the connective tissue 

 of muscles; they bind muscles to bones. The tendons 

 are sometimes very long, and attach a muscle to a bone 

 at a distance. This is true in the hand. The thick 

 muscles that move the hand are in the forearm, while the 

 hand itself is slender and skillful. The long tendons that 

 extend to the hand and the foot are held down, near the 

 bones, by means of circular bands (ligaments) at the 

 wrist and ankle. 



The bones act as levers (cf. 198 and 199). The fulcrum is the 

 joint. The weight is the member that is moved, and whatever it holds. 

 The power is the pull produced 

 by the end of the muscle or 

 tendon at the place where it is 

 attached to the bone. To lift 

 your forearm (Fig. 277) the biceps 

 muscle of the upper arm (Fig. 276) 

 contracts, and pulls upon the 

 radius bone a few inches below 

 the elbow. The power is be- 

 tween the fulcrum and the weight; 

 hence this is a lever of the third FlG 277. 



claSS. Most of the levers in the A Third Class Lever in the Arm. The power 

 . , < , i , i i ! i s supplied by the contraction of the biceps 



body are ot the third class. muscle. 



Some muscles are under the control of the will (vol- 

 untary muscles), and some are not (involuntary muscles). 



