322 PHYSIOLOGY. 



gree under the control of the will, yet it cannot be continued 

 long. This duration will be shorter in proportion as the 

 contraction is violent or moderate. f All muscles do not act 

 at the same time, for as some are contracting, such as the 

 flexors, or those that bend the limbs, the extensors, or those 

 that straighten them, are relaxed^ 



13 Pit is by the constant action of the musclesV that the 

 body is^kept in an erect position. If a person gets asleep 

 while sitting or standing up, the head 'falls forwards, and if 

 he did not wake, the body would fall likewise. The same 

 happens when a person is deprived of sense, by a fit of 

 apoplexy or palsy, or by the intemperate use of ardent 

 spirits. 



14. We see that it requires a long time for children to 

 learn to walk securely. fThe reason is, that the base of sup- 

 port for the body is small $ being only the space between 

 the feet and that on which the feet rest. The larger the 

 base, the easier it is to maintain an erect position ; so that 

 persons with small feet do not stand as firmly as those with 

 large ones. This accounts for the difficulty of standing on 

 our toes, or of walking on a rope. It is practice and in- 

 stinct which teaches children where to place their feet in 

 order to be most secure when erect ; their muscles also are 

 still weak, for want of exercise. 



15. Walking consists in a succession of steps. tWe first 

 balance the body on one foot, then bending the opposite foot 

 on the leg, and the leg on the thigh, we bend the thigh on 

 the pelvis, and so shorten the limb. In this way the leg is 

 brought forward, and the foot is then brought to the ground 

 resting first on the heel ; the body is then partially rotated 

 on the head of the thigh bone, and the other leg is raised, 

 bent, and carried forward in the same manner, and the foot 

 placed in advance of the other J The legs thus act as levers 

 to propel the body along, and the longer the levers the more 

 rapid will space be measured overA 



16. The utility of walkingj'excels that of all other modee 



