484 MANUAL OF PHYSIOLOGY. 



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

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

 the other. The vertical line from the centre of gravity of the 

 cranium must vary with every forward, backward, or lateral 

 movement of the head or neck, but in the erect posture it passes 

 a little in front of the axis of rotation of the atlanto-occipital 

 joint and somewhat behind the curve of the cervical vertebrae, 

 so that the head may be said to be poised on the apex of the 

 vertebral column, with some tendency to fall forward. There 

 are no ligamentous structures which can lock the joints so as to 

 keep the head in the erect position ; therefore, without the aid of 

 muscular force, the head will fall forward or backward, according 

 to the position it is in when the muscles become relaxed, as in 

 sleep. 



From the foregoing facts it will be seen that there exists a 

 kind of coordinated antagonism at work in ordinary easy standing 

 which keeps the elastic, pliable body upright, without the rigidity 

 adopted when standing "at attention." The muscular action is 

 more exercised when we are not on steady ground and varied 

 coordination becomes necessary ; for instance, when we go on 

 board ship for the first time. Station then takes some little time 

 to become perfected, and requires new associations of movement. 

 The gastrocnemius and soleus relax the ankle in a degree just 

 proportionate to the amount of flexion of the knee permitted by 

 the quadriceps extensor cruris, while simultaneously the great 

 gluteal muscle allows the body to incline forward so as to keep 

 its centre of gravity in the proper relation to the basis of support. 



WALKING AND RUNNING. 



The common act of progression is accomplished by poising the 

 weight of the body alternately on one leg called the supporting 

 limb and with the other the pendulous limb tilting the body 

 forward out of equilibrium, and then swinging the latter limb 

 forward and placing it in front so as to prevent the body falling 

 forward. In its turn, this then becomes the supporting leg. The 

 swinging leg is described as having two phases (1) active, while 

 pushing off from the ground ; and (2) passive, while swinging 



