316 THE VERTEBRATE BODY 



tracted without any resistance from opposing muscles. This resisting 

 force enables us to fix a joint in position as is done when we maintain 

 a standing position. 



To do all these things one would expect to find an extremely compli- 

 cated mechanism. Actually, the arrangement is quite simple. Each 

 muscle consists of a large number of muscle fibers; these are grouped 

 together in motor units. The nerve which innervates the muscle is com- 

 posed of many nerve fibers and one nerve fiber runs to each motor unit. 

 When only one of these nerve fibers stimulates the muscle, only its 

 particular group of muscle fibers is activated and only they contract. 

 Thus, by stimulating varying numbers of nerve fibers the brain can 

 regulate the number of motor units pulling at one time and consequently 

 the force which the muscle as a whole exerts. 



The way in which this system works may be illustrated by holding 

 a book in your hand. If your brain stimulates enough motor units you 

 will be able to exert enough of a pull to lift the book against gravity. 

 With slightly fewer motor units in action, you will just balance the pull 

 of the muscle against the force of gravity and will be able to hold the 

 book in place. If still fewer motor units are stimulated gravity will pull 

 the book down, but you will be able to lay it down gently on a table due 

 to the resisting action of the muscle fibers that are being stretched while 

 they are being stimulated to contract. Finally, if no motor units are 

 being stimulated, your hand and the book will drop with a bang in the 

 absence of any resisting force. 



The movement of the body that results from such contraction is called 

 the action of the muscle. Several muscle actions might be mentioned to 

 illustrate this principle. Limb muscles which bend a part of the limb 

 are flexors. Opposing muscles that straighten the limb out are exten- 

 sors. Muscles which pull a limb away from the median body line are 

 abductors, while those that pull it back toward the median line are ad- 

 ductors. Muscles that rotate a body part are called rotators, some of 

 which move the structure clockwise and others move it counterclockwise. 



There are always such opposing sets of muscles that do just the oppo- 

 site things and purposeful movements of the body are only possible 

 through muscle coordination, with just the proper degree of resistance 

 and relaxation being exhibited by one set of muscles when its opposing 

 set is contracting to move the body part. 



If you have ever seen frog legs cooked, you were probably amazed at 

 the activity of the legs when they were put in hot grease ; they will al- 

 most jump out of the skillet. This is because the cells live for a long 

 time after the frog is technically dead as an individual. Because of this 

 characteristic, the muscles of frogs are ideal to demonstrate principles of 



