THE MUSCULAR SYSTEM. 23 



_ v Some of the muscles of the body act 



Two Kinds 



of Muscular only when they are directed to do so by 

 the mind or the will, These are called 

 voluntary muscles. Others act without being con- 

 trolled by the will. These are called involuntary 

 muscles. The muscles of the arm, for example, are 

 voluntary muscles. The muscles which produce the 

 action of the heart are involuntary. Some muscles 

 may act either with or without the action of our will. 

 For instance, the muscles which produce winking 

 usually " wait for no thinking." But we may will to 

 wink, and wink whenever we please. On the other 

 hand, the will usually controls the action of the 

 motion of the jaws. But sometimes, as in the case 

 of a chill, these muscles produce chattering of the 

 teeth rather contrary to the direction of the will. 

 HOW the Motion is produced by the muscles, by 

 the contraction of the fibers. A muscle 



Act. 



shortens more or less according to the 

 degree of motion which it is to produce. The short- 

 ening in length is caused by a swelling out of the 

 muscles sideways. This swelling or bulging of a 

 muscle can easily be perceived while it is contracted 

 and pulling or holding the part which it moves. 

 Grasp your arm between the elbow and shoulder 

 firmly between your thumb and fingers. Now raise 

 your forearm toward your shoulder; you feel the 

 thickening of the muscle which raises your arm. 

 This (84) is the muscle whose action you so plainly 

 feel. It is called the biceps muscle of the arm. This 



