ADJUSTMENT OF THE CENTRE OF GRAVITY. 399 



gravity on one side of the natural centre of the base ; another 

 curve is formed by a muscular action, in a sound part of the 

 spine, to counteract the first, and keep the centre of gravity in 

 its natural position. 



The perception of a tendency to fall, when the centre of grav- 

 ity is in a wrong situation, first induces us to make efforts to 

 resist this tendency ; we learn by experience what these efforts 

 ought to be : and by habit we at length make them without 

 consciousness. 



As the natural tendency of the skeleton, when we stand, is 

 to bend at the articulations, and, therefore, to fall forwards; the 

 muscles which have the principal effort in keeping the body 

 erect, must be the extensors. 



Thus, the muscles on the back of the leg, and particularly 

 the soleus, keep the tibia erect : while the muscles on the front 

 of the thigh, the vasti and crureus, produce the same effect 

 upon the os femoris : the bones being kept steady by the occa- 

 sional counteraction of the antagonist muscles. 



The whole lower limb is thus made erect by an exertion 

 which begins at the foot, while the foot is fixed to the ground 

 by the weight and pressure of the body above it. 



The trunk of the body has a strong tendency to bend for- 

 ward at the articulations of the thigh bones and the ossa 

 innominata. This tendency is resisted by the muscles which 

 lie on the back part of the ossa femoris, and extend the trunk 

 on those bones, viz. the glutei maximi. 



The muscles which arise from the tuberosity of the ischium, 

 and are inserted into the leg, the semitendinosus, semimembra- 

 nosus, and the long head of the biceps flexor cruris, have also 

 this effect. 



The flexure of the thoracic and lumbar portions of the spine 

 is counteracted by the sacro-lumbalis, and longissimus dorsi, 

 which act from the sacrum and back parts of the pelvis. The 

 yellow ligaments, which are elastic, must also co-operate to 

 this effect : so that with regard to the spine, there is an additional 

 agent distinct from the muscular power. 



Indeed, respecting the vertebral articulations in general, it 



