FORCES OPERATIVE DURING PROGRESSION. 273 



balances the vertical weight of the body through the leg, and the 

 horizontal force (ad) balances the backward slipping of the foot which 

 would otherwise occur. 



The forces which we have now considered may be termed external ; 

 and in contradistinction to these are the internal forces of the body, 

 the friction occurring at the joints, the tension of ligaments, tendons, 

 and over-stretched muscles; and, over and above all these, the force 

 of muscular contraction. In all forms of progression muscular con- 

 traction is that which opposes the external forces which act upon 

 the body. 



VOL ii. 18 



