118 The Vertebrate Organ Systems 



type of lever. In man, the up and down movement of the head on the 

 vertebral column exemplifies this. In the levers of the second class, 

 the weight is in the center, with the fulcrum at one end and the power 

 at the other. A wheelbarrow illustrates this ; but this type of leverage 

 is not found in animals. The commonest type of leverage found in 

 animals is that illustrated by the levers of the third class. In this 

 type, the power is in the center, with the weight at one end and the 

 fulcrum at the other. Nearly all arm and leg movements utilize this 

 type of leverage. This allows a very small contraction at one end of 

 a muscle to bring about relatively large movements at the other end. 

 Other types of movements not involving leverage are also found in 

 animals. In some of these, direct force is applied. 



Inasmuch as a muscle can bring about movements only by contract- 

 ing and pulling, any movement involves a minimum of two muscles 

 which thus act as an antagonistic pair. When the forearm is drawn up- 

 ward, the muscle on the medial surface of the upper arm contracts 

 while that on the lateral surface relaxes. To lower the forearm, the 

 opposite action must take place. Actually most actions involve groups 

 of antagonistic pairs rather than single muscles. Those groups of 

 muscles which work together to bring about a single movement are 

 known as synergists. 



Types of Movements. — As muscles do work in antagonistic pairs, 

 each individual muscle or synergistic group can perform but a single 

 movement. These movements which are dependent upon the type of 

 joint involved have been classified in the following manner : | 



Flexion: A movement in which the angle between two adjacent parts 



is decreased. 

 Extension : A movement in which the angle between two adjacent parts 



is increased. 

 Depression : A. movement in which a part is drawn downward. 

 Elevation : A movement in which a part is drawn upward. 



Adduction: Movement toward the median line of the body. 



Abduction : Movement away from the median line of the body. 



Rotation: Movement of an appendage or a part upon its long axis. 



This may be medial or lateral. 

 Circumduction: Movement on a pivot in which a circle is described. M 



Pronation: The movement by which the palm of the hand faces down-" 



ward. 

 Supination : The movement by which the palm is turned upward. 

 Constriction : A movement which reduces the size of an opening. 

 Dilation: A movement which increases the size of an opening. 



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