LOCOMOTION 



343 



of circles. This movement is described as circumduction. In 

 this latter connection Leonardo da Vinci, more than four centuries 

 ago, made the following remark : " The movements of the 

 shoulders are infinite. If the arm is caused to sweep out a circle, 

 all the movements which are possible for that shoulder are thereby 

 made. 



Furthermore, as any continuous quantity is divisible to infinity, 

 the circle thus swept out is itself a continuous quantity due to 

 the movement of the arm, which latter cannot produce a con- 

 tinuous quantity unless it is itself continuous. Hence the move- 

 ment of the arm, having passed through every part of a circle, and 

 the circle being divisible to infinity, the variations (of movement) 

 of the shoulders are also infinite." ( x ) 



The degree of liberty of movement of a limb depends on the 

 nature and form of the joint. For joints of the same type the 

 speed and amplitude of move- 

 ment are determined by the 

 length of the arm of the lever, 

 the power, and the resistance. 

 We know that the levers of 

 locomotion are almost entirely 

 of the third order ( 87). 



In fig. 243 the planes in 

 which movement takes place 

 are shown : 



(a) The frontal, X'ZXZ'. 



(b) The horizontal, XY'X'Y 



(c) The sagittal (or anterior- 

 posterior) YZY'Z'. 



Every movement can be referred to these three planes. 



Adduction and Abduction are, however, referred to the median 

 plane of the body which lies in the sagittal plane. 



265. To simplify the study of the movements of the limbs the 

 following assumptions must be made : 



(a) That the longitudinal axes of any limb passes through its 

 centre of gravity, and also through the centre of its joint (Otto 

 Fischer found that this is, actually, approximately correct.) 



(6) That any joint may be considered as having one or more 

 definite and fixed axes. (In practice these axes are often subject 

 to slight displacements from their initial positions). 



FIG. 243.. 



Plan of the orientation of the 

 limbs of man. 



Leonardo da Vinci (Trattato della Pittura, p. 107 (Milan, 1804). 



