224 THE PHYSIOLOGY OF MUSCULAR WORK 



THE EFFECTS OF EXERCISE UPON THE MUSCULAR SYSTEM 



The muscles of the body are in a condition of partial contraction 

 even during apparent rest. This so-called tone is related to the 

 motor and sensory nerves which supply the muscle. If these 

 nerves be divided and do not regenerate the muscles will first 

 waste or atrophy, owing to disuse, and then degenerate. De- 

 struction of the motor cortex of the brain will cause atrophy of 

 the muscles, but they do not degenerate, for through their motor 

 and sensory nerves they are still under the influence of the spinal 

 cord. 



The tone of the muscles is diminished by sleep ; the observant 

 mother knows when her child has fallen to sleep in her arms by 

 the relaxation of its body. A far greater decrease in tone is pro- 

 duced by the action of anaesthetics ; the arm of a patient drops 

 in a limp and lifeless manner when he is deeply anaesthetised. 



From time to time the tone of the muscles varies in response 

 to impulses from the skin and other parts of the body ; it is in- 

 creased by cold, diminished by heat, and herein appears to lie the 

 chief explanation of the effects of different climates. The body is 

 invigorated by a bracing atmosphere, it is depressed by a relaxing 

 one. The tone of the muscles is accompanied by chemical changes 

 and the production of heat ; in a stagnant moist and warm atmos- 

 phere the body does not lose heat rapidly, and therefore in the 

 maintenance of a constant internal temperature produces less heat 

 by diminishing the activity of its muscles, and as far as possible 

 favours the loss by a dilatation of the blood vessels of the skin. 

 The opposite effects are produced by a cold dry atmosphere, or even 

 by warm air if it be sufficiently dry and in motion. European 

 children afford a good instance of these influences ; they will not 

 flourish in the hot moist districts of India. Two more examples 

 will be sufficient. The value of a cold bath on getting up in the 

 morning lies not in its cleansing properties so much as in the 

 bracing or stimulating effect which it has upon the whole muscular 

 system, including the muscles of the cutaneous arterioles. The 

 contrast between the walk of a man on a cold frosty morning and 

 on a damp warm day is known to every one who lives in this 

 country. 



The tonic contraction of the muscles may be so greatly increased 



