

WALKING AND RUNNING. 481 



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 atlan to-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 forwards. 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 mus- 

 cular force the head will fall forwards or backwards, 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 co- 

 ordination becomes necessary ; for instance when we go on board 

 ship for the first time. Station then takes some little time to be- 

 come perfected, and requires new associations of movement. The 

 gastrocnemius and soleus relax the ankle in a degree just propor- 

 tionate 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 forwards so as to keep its cen- 

 tre 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 

 forwards out of equilibrium, and then swinging the latter limb 

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

 forwards. 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 for- 

 wards like a pendulum. In starting one foot is placed behind 

 the other, so that the line of gravity lies between the two, the 



