50 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



( 1 ) If a sufficient interval is allowed between contractions no fatigue 

 is apparent. With a load of 6 kilograms, for instance, the flexor 

 muscle (M. flexor digitorum suhlimis) showed no fatigue when a 

 rest of 10 seconds was given between contractions. (2) After 

 complete fatigue with a given load a very long interval (two 

 hours) is necessary for the muscle to make a complete recovery 

 and give a second record as extensive as the first. (3) After 

 complete fatigue efforts to still further contract the muscle 

 greatly prolong this period of complete recovery, — a fact that 

 demonstrates the injurious effect of straining a fatigued muscle. 

 (4) The power of a muscle to do work is diminished by conditions 

 that depress the general nutritive state of the body or the local 

 nutrition of the muscle used; for instance, by loss of sleep, 

 hunger, mental activity, anemia of the muscle, etc. (5) On the 

 contrary, improved circulation in the muscle — produced by 

 massage, for example — increases the power to do work. Food 

 also has the same effect, and some particularly interesting 

 experiments show that sugar, as a soluble and easily absorbed 

 foodstuff, quickly increases the amount of muscular work that 

 can be pertormed. (0) The total amount of work that can be 

 obtained from a muscle is greater with small than with large loads, 

 since fatigue sets in more rapidly with the larger loads. (7) 

 Marked activity in one set of muscles — the use of the leg muscles 

 in long walks, for example — will diminish the amount of work 

 obtainable from other muscles, such as those of the arm. It is 

 very evident that the instrument may be used to advantage in the 

 investigation of many problems connected with gymnastics, diet- 

 etics, stimulants,* medicines, etc. 



Sense of Fatigue. — It should be noted in passing that in con- 

 tinued voluntary contractions we are conscious of a sense of fatigue 

 which eventually leads us, if possible, to discontinue our efforts. 

 This sensation must arise from a stimulation of sensory nerve fibers 

 within the muscle or its tendons, and it may be regarded as an 

 important regulation whereby we are prevented from pushing our 

 muscular exertions to the point of "straining." 



Muscle Tonus. — In addition to the sudden change in length 

 due to single or repeated stimuli, skeletal muscle exhibits another 

 kind of shortening designated under the term of tonus. The 

 characteristics of tonus or tonic contractions are that the shorten- 

 ing is very gradual, that it is or may be maintained for long periods 

 of time, and that it is effected apparently with very little consump- 

 tion of energy, the underlying chemical reactions which may be 



* Schumberg, "Archiv f. physiol.," 1899, suppl. volume, p. 289, and 

 Palmen, "Skandinavisches Archiv fiir Physiologic," 1910, 24, 168, 197. 



