THE PHENOMENON OF CONTRACTION. 51 



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 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. 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. The underlying cause 

 of tone is poorly understood. It may be, as implied above, simply 

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

 sub-minimal stimuli, or it may be an order of contraction quite 

 different from the usual visible movements; that is to say, the 

 shortening in the case of tonus may be due to a substance or mech- 

 anism in the muscle-fibers different from that which subserves the 

 ordinary quick movements which we designate as contractions. 

 However this may be, the fact of muscle tone is important in 

 a number of ways. It is of value, without doubt, for the normal 

 nutrition of the muscle, and, as is explained in the chapter 

 on Animal Heat, it plays a very important part in controlling 

 the production of heat in the body. The extent of muscle 

 tone varies with many conditions, the most important of which, 

 perhaps, are external temperature and mental activity. With 

 regard to the first, it is known that, as the external temperature 

 falls and the skin becomes chilled, the sensory stimulation thus 



