I04 THE COMMON FROG. [chap. 



animals, as in man, the muscles of the limbs belong 

 to different categories named from the kinds of 

 motion to which their contractions give rise. 



Thus, when two bones are united by a movable 

 joint (as the thigh-bone and shin-bone), muscles which 

 by their contraction tend to make the angle formed 

 by such bones acute are termed "flexors." Those, 

 on the contrary, which tend to open out such an angle 

 are termed *' extensors." 



In the fore-arm of man, and allied animals, there 

 are muscles which tend by their contraction to place 

 the hand in a position either of pronation or of 

 supination. 



When the arm and hand hang down, the palm being 

 directed forwards, the position is that of supination, 

 and the bones of the fore-arm are situate side by side. 



When the arm and hand hang down, but the 

 back of the hand is turned forwards, the position 

 is that of pronation, and the radius crosses over 

 the ulna. When we rest on the hands and knees, 

 with the palms to the ground, the fore-arms are in 

 pronation. 



Muscles which tend to place the fore-arm aiid hand 

 in the position of pronation are termed pronators ; 

 those which, by their contraction, tend to render it 

 supine are called supinators. 



It is somewhat surprising to find in an animal so 

 nearly related to fishes as McnobrancJms definite 

 flexors, extensors, pro- and supi-nators essentially 

 like those of higher animals ; and these distinctions 

 once established, persist up to man himself with 

 increasing complications. 



