76 PHYSICAL EXPRESSION. 



any attempt can be made at analysis or classifica- 

 tion; and we must keep a record of the move- 

 ment before us while we seek explanation of the 

 phenomena and try to understand what is expressed 

 by the movement. 



After making an anatomical description of the 

 movement for the sake of keeping it before us as a 

 mental account of the transient phenomenon, we 

 may apply certain physiological principles to the 

 analysis of the record of the movement. 



It is well known to physicians that there is much 

 practical difference in the signification of move- 

 ments in larger parts of the body of man in contrast 

 with movements seen in the smaller parts* It is 

 necessary in a compound movement a general 

 movement compounded of the movements of dif- 

 ferent parts or subjects to distinguish in descrip- 

 tion the action of small parts, such as the muscles of 

 the eyes, face, fingers, from movements of larger 

 parts, such as the skull, the shoulder, elbow or 

 wrist, the hip or the knee. This principle of the 

 different physiological signification of the action of 

 small and large parts will be illustrated in chaps, 

 vii. and viii. ; so also with the other physiological 

 principles. 



I think there is a considerable importance in the 

 two next principles, though I believe they have 

 been less generally studied than they ought to be. 

 These principles I name " Interdifferentiation ' ' and 

 " Collateral Differentiation." 



The term " interdifferent iatwn of movements " is 



* See chap. vii. " Small parts " are most affected in liemiplcgia. 



