74 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



metabolism, but the nature of this alteration is unknown at present. 

 There are, however, certain immediate changes in the conclition of 

 the muscle due to exercise which can be determined and which have 

 an influence upon its nutrition. There is, first, what we may call 

 the circulation factor. Exercise causes an increase in the blood- 

 flow in the muscle. Both the small arteries and the capillaries 

 become dilated, with the result that the volume of blood flowing 

 through in activity is much greater than in rest. This increased 

 flow provides the additional material needed for the processes of 

 restoration and possibly contributes directly to the growth of the 

 muscle. Secondly, we have the factor of tonus. Exercise leads 

 to an augmentation of tonus, while unexercised muscles become 

 soft and flabby. As stated on p. 51, it is not known at present 

 whether the underlying chemical changes in tonus are of the same 

 character as those occurring in contraction, but such evidence as 

 we have indicates that they are different and act upon a different 

 mechanism or structure in the muscle. These tonus changes are 

 essential to the maintenance of a normal nutritive condition, and it 

 may be assumed that when they are increased the efficiency and 

 growth of the muscle are facilitated. 



The changes caused in the circulation in the muscle by exercise 

 can be imitated in some degree by the action of massage, and in cor- 

 respondence with this result it can be shown that massage improves 

 the working power of a muscle and favors prompt recovery from the 

 effects of fatigue. But in a paralyzed muscle it has been shown 

 that massage alone does not suffice to protect the muscle from de- 

 generative atrophy. The normal tonic stimuli through the motor 

 nerve fibers are apparently essential for the maintenance of normal 

 metabolism. Massage of a muscle when its nervous connections 

 are intact leads to some increase in tonus as well as to an im- 

 provement in the circulation, and to the extent that it brings about 

 these reactions it may be assumed that massage acts as a partial 

 substitute for muscular exercise. Systems of exercise have been 

 developed with the intention of maintaining or improving the 

 condition of the muscles or for ensuring their symmetrical devel- 

 opment. Sports and games play an important role in these sys- 

 tems, particularly for the young, and the beneficent influences of 

 such activities are not limited to the muscles themselves or to the 

 neuromuscular apparatus involved in voluntary movements. 

 On the contrary, a large part of the value of exercise is its stimu- 

 lating effect upon the organs of the cardiovascular system, the 

 respiratory system, and upon the metabolic processes of the body 

 in general. In modern times two general systems of exercise have 

 been devised for use in schools, in armies, and in civil life in general 

 with the object of counteracting the evil results of the sedentary 



