MUSCULAR EXERCISE. 139 



peatedly continued until exhaustion it does harm; the period 

 of repair is not sufficient to allow replacement of the parts 

 used in wofk, 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 be- 

 cause muscular effort greatly increases the work of the heart, 

 not only 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 repre- 

 sented 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 any previous train- 

 ing. 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 represented 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 true also 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 em- 

 ploys equally or proportionately all the muscles: and hence 

 gymnnasia in which various feats of agility are practiced, 

 so as to call different parts into play, have attained a great 

 popularity. It should be borne in mind however, that the 

 legs especially need strength; while the upper limbs, in 

 which delicacy of movement, as a rule, is more desirable 

 than power, do not require such constant exertion; and the 



