154 THE HUMAN BODY. 



nerves. In such cases the muscles may themselves be per* 

 fectly healthy at first, but lying unused for weeks they become 

 altered, and finally, when the nervous injury has been healed, 

 the muscles may be found incapable of functional activity. 

 The physician therefore is often careful to avoid this by exer- 

 cising the paralyzed muscles daily by means of electrical 

 shocks sent through the part, while at the same time he tries 

 to restore the nerves; passive exercise, 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. Exer- 

 cise, 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, not merely because 

 more blood has to be sent to the muscles themselves, but they 

 produce great quantities of carbon dioxide, which must be 

 carried off in the blood to the lungs for removal from the 

 Body, and the heart must work harder to send the blood faster 

 through the lungs, and at the same time the breathing be 

 hastened so as to renew the air in those organs faster. The 

 least evil result of throwing too violent work on the heart 

 and lungs in this way is represented by being " out of 

 breath," which is advantageous insomuch as it may lead to a 

 cessation of the exertion. But much more serious, and 

 sometimes permanent, injuries of either the circulatory or 

 respiratory organs may be caused by violent and prolonged 

 efforts without due previous training. 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 rep- 

 resented by a daily walk of five miles. 



Varieties of Exercise. In walking and running the 



