i 



528 A TEXTBOOK OF PHYSIOLOGY 



ments which lock together the femur and the tibia, the knee can 

 neither be overextended nor bent to one side. In a man standing at 

 attention the plummet-line drawn from the centre of gravity passes 

 in front of the line joining the two ankle-joints ; the body is prevented 

 from falling forwards by the action of the calf muscles. The weight 

 of the body thus transmitted is borne by the spring of the arch of the 

 foot ; the balls of the toes and the heel rest upon the ground. 



The centre of gravity of the body is always kept over the base of 

 support by varying the attitude of the body. Thus a man stoops 

 when carrying a child on his back, but walks erect if it be on his 

 shoulders. If the child be on his arm he leans back, and to the other 

 side. In most of the Herculean feats of strength exhibited on the 

 stage, the strong man supports enormous weights, not by muscular 

 effort, but by so placing his body that the bones form pillars of support 

 on which the weight rests. 



In young children the centre of gravity is high, for the head is 

 large and the small feet form but a narrow base. A slight push from 

 behind brings the centre of gravity outside the base, and the child 

 must move its feet quickly forward or fall. Thus the tiny child has 

 many tumbles, for the brain has to learn by experience how to carry 

 out rapidly the appropriate movements. The younger a child the 

 more he tends to stand with his feet wide apart. The tottering old 

 man also widens his base of support by using a staff. 



The body is equilibrated by means of the proprioceptive mecha- 

 nism of the body (see p. 654) and the co-ordinating influence of the 

 cerebellum and the cerebrum. 



Walking. On standing on one foot the body is inclined to that 

 side, so that the other leg is left free to move. In walking, one leg, 

 say the right, is slightly bent at the knee and planted down in front 

 of the other. The weight of the body is thrown on this leg, while the 

 left leg, raised on the toes by the action of the calf muscles, forms a 

 straight stiff rod. The left leg, by giving a push to the ground, ne^t 

 throws the body forwards. Thereupon the right leg straightens up, 

 while the left, slightly bent at the knee, swings forward as a pendulum 

 and comes down in front of the right. It is now the turn of the right 

 leg to push off, and of the left leg to bear the weight of the body. 

 The length and rapidity of the step in walking naturally depend on 

 the length of leg. A duck waddles, a hen runs. The longer a pen- 

 dulum the slower it swings. Thus it is difficult for a long and a short 

 man to keep pace, and a regiment cannot maintain a regular march 

 when the men are fatigued, for each soldier then falls into his own 

 natural swing. 



Running. In running, both legs momentarily leave the ground. 

 The muscles act far more powerfully than in walking. The body is 

 raised and thrust forward, not only by the contraction of the calf 

 muscles of the hind-leg, biit by the powerful action of the extensors 

 of the thigh, which straighten the bent knee of the forward leg. The 

 bodv thus propelled forwards leaves the ground. Avhile the hind-leg 



