THE PHENOMENON OF CONTRACTION. 51 



the motor end-plates in the muscle, etc. The weak link in this chain, the ele- 

 ment that first exhibits fatigue in voluntary movements, has not been precisely 

 determined. 



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 eflorts. 

 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 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 amount, the 

 result being that the muscles enter into a state of contraction 

 which, while slight in extent, is more or less continuous. Ac- 

 cording to this view, the whole neuromuscular apparatus is 

 in a condition of tonic activity, and this state may be referred 

 in the long run to the continual inflow of sensory impulses into 

 the central nervous system. That is, the tonus of the skeletal 

 muscles is not only dependent on the nerve centers (neurogenic), 

 but is in reality an example of reflex stimulation of these centers. 

 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. Several 

 authors (Boeke, de Boer*) have claimed that in the reflex tonus of 

 muscle the efferent path is through the thoracic autonomic system 

 rather than through the cerebrospinal nerves. Some anatomical 

 evidence has been collected to show that skeletal muscle does, in 

 fact, receive a double motor innervation, a direct innervation 

 through the motor fibers of the cranial or spinal nerves, and an 

 indirect one through autonomic nerves. Section of these latter 

 in the rami communicantes is said to destroy tonus in the muscles 

 involved, and it is suggested that reflex tonus is mediated through 

 these fibers just as reflex contractions are mediated through the 

 medullated motor fibers of the cranial or spinal nerves. The 

 underlying cause of tone is poorly understood. It may be simply 

 a condition of subdued tetanus due to a constantly acting series of 

 * de Boer, "Zeitschrift fur Biologic," 1915, 65, 239. 



