148 COJIPAIUSOXS 01" STKUCTCKE IN ANIMALS. 



almost iipriglit iu their common attitude, at 

 rest, Avitli tlie head drawn back and laid be- 

 t-vveen the shoulders. Most birds during 

 repose throw the head back, bury it in the 

 feathers beneath the wing, and support them- 

 selves on their perch by means of one leg only. 

 Thus they regulate the position of the centre 

 of gravity. 



On the contrary, during flight, the bird 

 stretches forward its neck, in order to keep its 

 balance true, that is, to bring the centre of 

 gravity under the origin of the wings. In 

 some very long-necked birds, as the heron, the 

 neck is folded during flight, and the spear-like 

 bill directed forwards. This use of the neck 

 in balancing the body is precisely the same as 

 that of the arms, imder various circumstances. 

 If W'e stand on cue leg, and bend the body to 

 the right, the left arm will be involuntarily 

 extended, from an instinctive perception that 

 by so doing the balance will be maintained. 

 When children in play hop on one leg this 

 management of the arms in preserving an 

 equilibrium may be seen in contiriual opera- 

 tion. This involuntary and instinctive action 

 is the result of a peculiar " muscular sense,'' 

 which leads us to adjust th^ frame so as lo 



