480 MANUAL OF PHYSIOLOGY. 



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 backwards. 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 forwards. This tendency, however, is pre- 

 vented by the powerful glutei muscles, which also enable us to 

 regain the erect posture after bending the trunk forwards. 



The motions of which the pelvis and vertebral column are capable 

 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 yielding and 

 elastic, but practically jointless, pillar, the upper part of which 

 can alone be bent to such an extent as to require mention in dis- 

 cussing 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 asso- 

 ciated with the movements between the other cervical vertebra}. 



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

 column, and rests immediately in a kind of socket, which can 

 move as a double hinge around two axes at right angles one to 



