112 THE HUMAN BODY 



little finally being left but the connective tissue of the organ and 

 a few muscle-fibers filled with oil-drops. This is well seen in cases 

 of paralysis dependent on injury to the nerves. In such cases 

 the muscles may themselves be perfectly healthy at first, but ly- 

 ing unused for weeks they become altered, and finally, when the 

 nervous injury has been healed, the muscles may be found in- 

 capable of functional activity. The physician therefore is often 

 careful to avoid this by exercising the paralyzed muscles daily 

 by means of electrical shocks sent through the part; passive exer- 

 cise, as by proper massage, is frequently of great use in such cases. 

 The same fact is illustrated by the feeble and wasted condition of 

 the muscles of a limb which has been kept for some time in splints. 

 After the latter have been removed it is only slowly, by judicious 

 and persistent exercise, that the long-idle muscles regain their 

 former size and power. The great muscles of the " brawny " arm 

 of the blacksmith or wrestler illustrate the reverse fact, the 

 growth of the muscles by exercise. Exercise, however, must be 

 judicious; repeated frequently to the point of exhaustion it does 

 harm; the period of repair is not sufficient to allow replacement 

 of the parts used in work, and the muscles thus waste under too 

 violent exercise as with too little. Rest should alternate with 

 work, and that regularly, if benefit is to be obtained. Moreover, 

 violent exercise should never be suddenly undertaken by one 

 unused to it, not only lest the muscles suffer, but because muscular 

 effort greatly increases the work of the heart. No general rule 

 can be laid down as to the amount of exercise to be taken; for a 

 healthy man in business the minimum would perhaps be repre- 

 sented by a daily walk of five miles. 



Varieties of Exercise. In walking and running the muscles 

 chiefly employed are those of the lower limbs and trunk. This is 

 in part true of rowing, which when good is performed much more 

 by the legs than the arms: especially since the introduction of 

 sliding seats. Hence any of these exercises alone is apt to leave 

 the muscles of the chest and arms imperfectly exercised. Indeed, 

 no one exercise employs equally or proportionately all the muscles : 

 therefore gymnasia in which various feats of agility are practised, 

 so as to call different parts into play, have very great utility. It 

 should be borne in mind, however, that the legs especially need 

 strength; while the upper limbs, in which delicacy of movement, 



