FORCES ACTING OBLIQUELY UPON A BONE. 



253 



diminishing resistance. It must be borne in mind that the direction of 



traction may be greatly modified by the 



tuberosities over which the tendon may 



pass before insertion. A calculation of 



the moments at the ankle-joint will serve 



to illustrate the important principle 



alluded to above. The calculation is 



only true for the position at which the 



calculation is made, and the reason is 



that the moments vary with every change 



in position. 



Knorz l and Henke 2 pointed out 



that before the heel is raised the body 



is instinctively thrown forwards, so that 



the centre of gravity falls over the base 



of the toes on which the body rests, 



before the heel actually leaves the 



ground. As Bedart 3 pointed out, it 



is on this account that when we stand 



with our toes touching a vertical wall 



it is impossible to rise on the toes 



(Fig. 140). When in this position we 



lean forwards against the wall, the 



centre of gravity plumbs over a point 



midway between the ankle and the 



base of the toes, the wall prevents the 



experimenter from leaning forward 



enough to throw the centre plumb over 



the toes. He has therefore no support for his body and cannot raise it. 



Thus, when we raise 

 ourselves higher and higher 

 on the toes, the centre 

 of gravity shifts slightly 

 forwards towards the tips 

 of the toes. One can 

 assure oneself of this fact 

 by standing in front of 

 the door, with the toes 

 about 2 in. from the door. 

 If one can just raise the 

 heel, say half an inch, and 

 keep it in that position 

 without falling backwards, 

 it will be found impossible 

 to raise the body to its 

 full height. This can, 

 however, readily be done 

 on standing back, say half 

 an inch. Still more im- 

 portant is the fact that 



Fio. 140. — In a the tip of the toes, 

 the front of the chest, and the 

 chin are in the same vertical plane. 

 In b the individual rises on his 

 toes, and in so doing the body 

 moves forwards into the position 

 indicated. Were a wall to occupy 

 the position of the vertical line, 

 this action would be impossible. 



a 



^y 



2 



^ 



^ 



v 



141. — Diagram to represent the moments around 

 the ankle-joint (b) resting on the ground, and when 

 it is raised to the position b' '. a marks the centre 

 of the hall of the great toe, and c and c' the point 

 of insertion of the calf muscle. 



1 << 



Ein Beitrag zur Bestimmung der absoluten Muskelkraft," Marburg, 1865. 

 ; Ztschr.f. rat. Med., Leipzig u. Heidelberg, 1865, Bd. xiv. 

 6 Compt. rend. Soc. dc biol., Paris, 1892. 



