478 



MANUAL OF PHYSIOLOGY. 



FlG. 194. 



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 hurnerus upon 

 which is fixed the body or weight (mid- 

 dle 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 dia- 

 gram). 



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 ioint or 



Diagrams snowing the mode ... . 



of action of the three orders of in combination with other different mus- 

 levers (numbered from above cleg have tot ally different actions, at one 



downward) illustrated by the . j 



action of the elbow joint. time being synergetic and at another 



antagonistic. Thus the sterno-mastoid 



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

 cranium backward or forward, 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 

 surfaces without the interposition of any cartilage. They are 



