THE PHENOMENON OF CONTRACTION. 49 



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 

 muscle tone as a state of continuous contraction, it may be 

 that the apparently uniform condition is only superficial; that, in 

 fact, this phenomenon is substantially only a minimal tetanus, due 

 to a series of feeble but discontinuous stimuli received through 

 the motor nerve, each of which stimuli sets up its own chemical 

 change in the muscle. 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 mus- 

 cle 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 

 produced acts upon the nerve centers and leads to an increased 

 discharge along the motor paths to the muscle. The tone of the 

 muscles increases and may pass into the visible movements of 

 shivering. By this means the production of heat within the body 

 is increased automatically. Similarly, an increase in mental 

 activity, so-called mental concentration, whether of an emotional 

 or an intellectual kind, leads, by its effect on the spinal motor 

 centers, to a state of greater muscle tonus, the increased muscular 

 tension being, indeed, visible to our eyes. 



The Condition of Rigor. When the muscle substance dies 

 it becomes rigid, or goes into a condition of rigor: it passes from 

 a viscous to a solid state. The rigor that appears in the muscles 

 after somatic death is designated usually as rigor mortis, since its oc- 

 currence explains the death stiffening in the cadaver. It is charac- 

 terized by several features: the muscles become rigid, they shorten, 

 they develop an acid reaction, and they lose their irritability to 

 stimuli. Whether all of these features are necessary parts of the 

 condition of rigor mortis it is difficult to say; the matter will be 



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