48 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



involved in a muscular contraction namely, motor nerve cell, 

 motor nerve fiber, and muscle fiber the first named fatigues most 

 easily, and that the ordinary fatigue curve obtained from the ergo- 

 graph does not represent pure muscle fatigue, but fatigue of the 

 neuromuscular apparatus as a whole, the point of complete fatigue 

 being reached in the neural component of the mechanism before 

 the muscle itself loses its power of contraction. This interpreta- 

 tion, however, is not entirely certain. Recent work by Wedenski 

 has called attention to the fact that in the neuromuscular apparatus 

 the motor end-plate is a sensitive link of the chain, and that, when 

 the nerve is stimulated strongly with artificial stimuli at least, this 

 structure falls into a condition (parabiosis) in which it fails to con- 

 duct the nerve impulse to the muscle. It may be, therefore, that 

 in sustained voluntary contractions the end-plate fails first, and 

 thus is directly responsible for the appearance of fatigue. This 

 view explains readily why in such conditions the muscle is still irri- 

 table to direct electrical stimulation. 



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 stimulus of sensory nerve fibers 

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

 tant regulation whereby we are prevented from pushing our muscular 

 exertions to the point of " straining." 



Muscle Tonus, In addition to the conditions of contraction 

 and of relaxation the living muscle exhibits the phenomenon of 

 "tone." By muscle tone we mean a state of continuous shortening 

 or contraction which under normal conditions is slight in extent 

 and varies from time to time. This condition is dependent upon 

 the connection of the muscle with the nerve centers, and we may 

 assume that under normal circumstances the motor centers are 

 continually discharging subminimal nerve impulses into the muscles 

 which cause chemical changes similar in kind to those set up by 

 an ordinary voluntary effort, but less in degree; the result being 

 that the muscles enter into a state of contraction which, while 

 slight in extent, is more or less continuous. According to this view, 

 the whole neuromuscular apparatus is in a condition of tonic ac- 

 tivity, and this state may be referred in the long run to the con- 

 tinual inflow of sensory impulses into the central nervous system. 

 The tone of any particular muscle or group of muscles may be 

 destroyed, therefore, by cutting its motor nerve, or less completely 

 by severing the sensory paths from the same region. If, for in- 

 stance, one severs in a dog the posterior roots of the spinal nerves 

 innervating the leg there will be a distinct loss of muscular tone, 

 although the motor nerves remain intact. While we speak of this 



