GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 73 



tability, may exert a protective influence and prevent the work from being 

 carried under ordinary conditions to the point of exhaustion. In favor of 

 this view he states that the muscles even of starving animals, although 

 incapable of long-continued work, do not make contractions of the tvpe 

 characteristic of fatigued muscles (see p. 115); on the other hand, muscles 

 which have been subjected to lactic acid, one of the waste products resulting 

 from muscular work, whether it be free or combined, as it probably is in the 

 muscle, with potassium or sodium, do make contractions of the type shown 

 by fatigued muscles. Waller 1 has of late laid much stress upon the action 

 of C0 2 to stimulate protoplasm when present in small amounts and to anaes- 

 thetize it when in larger quantities. C0 2 is also a waste product of muscle, 

 but it is doubtful whether the paralyzing effect of large amounts can be 

 regarded as a fatigue effect. 



Maggiora, in his experiments upon the fatigue of special groups of mus- 

 cles, likewise found that the taking of food causes only a partial recovery of 

 the tired muscles, and that an interval of rest is essential to complete recoverv. 

 In these experiments the irritability of the muscles was tested not only by 

 volitional impulses, but by the strength of the electric current required to 

 cause direct excitation. A curve of fatigue of human muscles by voluntary 

 contractions is shown in Fig. 59, and one resulting from electrical excitation 

 of the muscle in Fig. 58. In the case of vigorous men, one and a half hours 

 suffice to restore the muscles of the forearm which have been completelv tired 

 out by raising a heavy weight many times. He also observed that the time 

 required for recovery can be greatly shortened if the circulation of the blood 

 and lymph in the muscles be increased by massage. This suggests that the 

 power of the cell to give off its waste products to the blood is sufficiently 

 rapid to keep pace with the ordinary production, but not with the more rapid 

 formation taking place during fatiguing work. This would seem to be the 

 case in spite of the fact that circulation of the blood and lymph in the mus- 

 cles is increased during action. This increase in the circulation through the 

 acting muscle is brought about in part by the fact that the muscle massages 

 itself by its own contractions. It is a pumping mechanism, which acts at the 

 time when the increased taking of oxygen and nutriment and giving off of 

 waste products make the rapid renewal of the restoring fluids imperative. 

 Every time the muscle contracts the swelling, tense fibres compress the lym- 

 phatics and blood-vessels between and about them, and when it relaxes the 

 valves in the lymph vessels and veins prevent the return of the fluid which 

 has been squeezed out. In addition to this, when muscles are stimulated to 

 action by impulses coming to them from the central nervous system, the mus- 

 cles in the walls of the blood-vessels of the muscle are acted upon by their 

 vaso-dilator nerves, and, relaxing, permit a greater flow of blood through the 

 muscle ; when the muscles cease to be excited the muscles in the vessel walls 

 gradually regain their tone, and the blood-supply to the muscle tissue is 

 correspondingly lessened. This arrangement would seem to suffice for the 

 1 Lectures on Physiology, first series, on Animal Electricity, London, 1897, p. 47. 



