SPECIAL PHYSIOLOGY OF THE MUSCLES 207 



In standing erect the position of the body is such that the 

 centre of gravity is vertically over the base formed by the 

 feet, and when the limbs are placed against each other, the 

 longitudinal axis of the body is nearly vertical. 



The base of support is a hexagon whose angles are formed 

 by the heads of the first and fifth metatarsus and by the 

 calcaneum on both sides. The centre of gravity of the body 

 lies somewhat in front of the promontorium of the spinal 

 column. 



The muscles which during quiet standing fix the limb are, 

 in reality, only: 



1 . Calf-muscles. The contraction of these prevents the 

 body from falling forward which might be caused by the 

 bending of the lower part of the leg at the ankle-joint. 



2. The muscles of the neck, by the contraction of which 

 the forward sinking of the head is prevented (lowering of the 

 chin upon the chest as in sleep). 



3. To a smaller degree the neck and hip muscles, which 

 prevent the bending of the cervical and lumbar vertebrae. 



Besides this the limbs are held firm by the following 

 ligaments : 



(1) The superior ileo-femoral ligaments (ligamenta Ber- 

 tini) which prevent the body from falling backward by turn- 

 ing at the hip-joints. 



(2) The posterior crucial ligaments of the knee-joints, 

 which prevent the body and the upper part of the legs from 

 falling forward by turning at the knee-joints. 



As the arms hang loosely suspended by the sides of the 

 body, they need no mechanism for fixation. 



Many authors suppose that the body is not thrown forward but 

 backward by the bending of the knees. If this is true, the fixation 

 of the knees is not caused by the posterior crucial ligaments, but 

 by the quadriceps femoris. 



Turning the feet out aids in holding the lower limb against the 

 foot in the ankle, for in placing the feet outward the two axes of 

 the joints do not fall in the same direction but converge forward. 

 This makes a simultaneous rotation around both axes, without a 

 change in the position of the legs, impossible. 



