LOCOMOTION 341 



freedom which entails a greater degree of static activity on the 

 part of the muscles. 



263. (B) Dynamics of the Human Body. Whenever the 

 mechanical equilibrium of the body is disturbed movement com- 

 mences. This movement is the result of forces which may be 

 internal or external, and may be considered as concentrated at 

 the centre of gravity of the body ( 23). We know that the 

 interior forces do not affect the position of the centre of gravity. 

 If we imagine a man standing on a perfectly smooth surface and 

 assume that there is no friction between his feet and that surface, 

 he cannot change his position, but can only turn on himself. 

 The only motion possible is that of rotation around a vertical 

 axis on which the centre of gravity remains unchanged ( 27). 

 Thus if the arms are placed symmetrically and caused to describe 

 two circles having the same direction of rotation, the body will 

 thereby be caused to make a complete revolution in the opposite 

 direction. This cannot be effected in any other manner. If the 

 body is suspended by a rope the extension of an arm in one direc- 

 tion will only cause a corresponding retraction in the opposite 

 direction, f 1 ) The sum of the moments is necessarily always 

 zero. 



In regard to external forces, the case is different. Of these 

 the most important, as far as natural bodies are concerned, is 

 that of gravity. Also due attention must be given to friction 

 and air resistance. A man in falling describes a portion of a para- 

 bola, this being the trajectory of his centre of gravity. 



Fl. 241. 



Trajectory of centre of gravity of an animal (cat) thrown 

 horizontally. 



Marey ( 2 ) demonstrated by chronophotograp hy that the body of 

 an animal (e.g., a cat) can make a complete rotation under the 

 influence of interior (muscular) forces (fig. 241). Nevertheless 

 the general centre of gravity describes an arc of a parabola just 

 as if the force of gravity was acting on a rigid body. 



( x ) E. Kohlrauch, Physik des Turnens, p. 50, 1887. 



j 2 ) Comptes Rendus Sciences, vol. cxix., p. 714, (1894). 



