SKELETAL MOVEMENTS. 



475 



FIG. 194. 



around the trochlea which acts as the fulcrum. This is shown in 

 the upper diagram, in which the hand is striking a blow with a 

 dagger. 



The second order comes into play 

 when the hand, resting on a point of 

 support, acts as the fulcrum, and the 

 triceps pulling on the olecranon is the 

 power which raises the humerus upon 

 which is fixed the body or weight 

 (middle diagram). 



The third order may be exemplified 

 by the action of the biceps in ordinary 

 flexion of the elbow. Here the muscle, 

 which is the power, is placed between 

 the fulcrum represented by the lower 

 end of the humerus and the weight 

 which is carried by the hand (lower 

 diagram). 



The various groups of muscles, which 

 are so arranged as to assist each other 

 when acting together, are called syner- 

 getic, and those which when contracting 

 at the same time oppose each other, are 

 called antagonistic. The same muscles 

 may, in different positions of a joint or 

 in combination with other different 

 muscles, have totally different actions, 

 at one time being synergetic and at 

 another antagonistic. Thus the steruo- 



Diagrams showing the 

 mode of action of the three 

 orders of levers (numbered 

 from above downwards) il- 

 lustrated by the action of 

 the elbow-joint. 





mastoid muscle may, in different positions of the head, either 

 bend the cranium backward or forwards, and so cooperate with 

 two sets of muscles which are definitely antagonistic to one 

 another. 



JOINTS. 



The unions between the bones of the skeleton are very varied 

 in function and character. They may be classified as : 



1. SUTURES, in which the bones are firmly united by rugged 



