PHYSIOLOGICAL PRINCIPLES. 77 



used to indicate that the conditions of movements 

 are different in the large and in the small parts. 

 There may be a great difference observed, in the 

 upper extremity of a man's arm, in the amount of 

 movement, or in the number of movements of the 

 small parts as compared with the large parts. Thus, 

 in writing, painting, sewing, the fingers, knuckles, 

 and wrist do almost all the movements, there being 

 comparatively little action in shoulder and elbow : 

 here, then, is marked interdifferentiation of the 

 movements of the upper extremity. Contrast this 

 example with the use of a man's upper extremity 

 for ground-digging or boxing. In this case the 

 fingers and small parts are but little used, the move- 

 ment is from shoulder and elbow; here, again, inter- 

 differentiation of movements is marked in degree, 

 the large parts being more used than the small 

 parts. 



Collateral differentiation of movements, or a dif- 

 ference in the movements of collateral parts, is our 

 next principle. The knuckles and the fingers are 

 collateral parts: they are all equally small parts, 

 they can move all together or each can move sepa- 

 rately. There may be differences in the movements 

 of the fingers, or collateral differentiation of move- 

 ments may be seen. In writing, the movements of 

 the pen are effected almost entirely by the thumb, 

 index, and middle fingers : this is collateral dif- 

 ferentiation of the movements of the fingers in the 

 act of writing. 



Symmetry of movement in man and in animals 

 is by no means a new subject ; it has been noted 



