

ERECT POSTURE. 483 



of gravity passes just through, or very slightly behind, the axis 

 of rotation of the joint, so that, if anything, there is a slight 

 tendency for the knee to bend. This is completely checked, and 

 the body balanced by the powerful extensor muscles of the 

 thigh. 



3. In the hip joints, which have to support the trunk and head, 

 the line of gravity falls just behind the line uniting the joints 

 when the person is perfectly erect, so that here the body has a 

 tendency to fall backward. This is prevented by the strong ilio- 

 femoral ligament. When, however, the knee is not straightened 

 to the full extent, so that the line of gravity passes through or a 

 little behind the axis of rotation of that joint, then the pelvis is 

 very slightly flexed on the femora so that the axis of the joints 

 lies exactly in or a little behind the line of gravity, and thus the 

 body inclines rather to fall forward. This tendency, however, 

 is prevented by the powerful glutei muscles, which also enable 

 us to regain the erect posture after bending the trunk forward. 



The motions of which the pelvis and vertebral column are capa- 

 ble are too slight to deserve attention here. The vertebral 

 column, wedged in, as it is, between the two innominate bones, 

 may be taken, together with the pelvis, as forming a very yield- 

 ing and elastic, but practically jointless, pillar, the upper part of 

 which can alone be bent to such an extent as to require mention 

 in discussing the mechanism of station. 



The individual joints between the cervical vertebrae permit but 

 a slight amount of movement when taken separately, but by 

 their aggregate motion they enable considerable extension and 

 flexion of the neck to take place. These motions follow so closely, 

 and are so inseparably associated with those of the head on the 

 upper vertebra, that there is no need to consider them separately 

 from the latter. 



The atlanto-occipital joints admit of some little lateral move- 

 ment, but that in the antero-posterior direction is much the more 

 important, but even this would be insignificant were it not 

 associated with the movements between the other cervical verte- 

 brae. 



The cranium has then to be balanced on the top of a flexible 



