306 PHYSIOLOGY AND HEALTH. 



of oxygen, and, consequently, a more frequent respiration 

 during exercise. But if the lungs are unsound, or inflamed, 

 or in any way impaired, or if their motions are impeded by 

 any external incasement around the chest, they cannot re- 

 ceive air sufficient to carry off the excess of wasted particles 

 which are thrown into them. For this cause, a man laboring 

 under asthma or consumption cannot run, or pump at a fire- 

 engine, or mow grass, or perform any labor that requires 

 great and continuous effort. An abundant supply of pure air 

 is equally necessary to carry off the excess of waste. Men 

 who work in close shops, mines, and the holds of ships, have 

 less power than those who work in the fields. 



706. The heart, being the engine that propels the blood, 

 is required to work with greater force and rapidity when 

 exercise demands a greater supply of nutriment in the tis- 

 sues. If this organ is unsound, it cannot make this extra- 

 ordinary exertion ; and those who have diseased hearts cannot 

 perform very active labor. Mr. H., a very industrious farmer, 

 once complained to me " that he could not cut his wood, 

 nor mow his grass, as he had done, for the exertion imme- 

 diately caused great distress about the region of the heart, 

 and then he could not move." He was suffering from a dis- 

 ease of the heart, of which he afterwards died. 



707. The brain is connected with all the muscles through 

 the medium of nerves, and directs and sustains their motions. 

 The state and health of the brain and mind affect, very ma- 

 terially, the value of exercise and the power of labor. We 

 will, or determine, to move the finger ; at the same instant 

 the volition is sent from the brain to the muscle that moves 

 the finger ; this then contracts, and the finger moves. This 

 is the case with all voluntary motion ; yet the volition of the 

 mind and the action of the brain are not always observed. 

 Some motions are performed so much from habit, that we are 

 unconscious of the volition. When we walk, we contract the 

 muscles, those which bend the joint of the hip, the knee, and 

 ankle, of one side ; and, at the same moment, we contract the 

 muscles which straighten these joints on the other side. At 



