THE PHENOMENON OF CONTRACTION. 49 



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, as it were, 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, as it were, 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 fluid to a solid state. The rigor that appears in the muscles after 

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

 rence 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 

 discussed briefly below. Some of the facts which have been ob- 

 served regarding rigor mortis are as follows: After the death of an 

 individual the muscles enter into rigor mortis at different times. 

 Usually there is a certain sequence, the order given being the jaws, 

 neck, trunk, upper limbs, lower limbs, the rigor taking, therefore, a 

 descending course. The actual time of the appearance of the rigidity 

 varies greatly, however; it may come on within a few minutes or a 

 number of hours may elapse before it can be detected, the chief de- 

 termining factor in this respect being the condition of the muscle 

 itself. Death after great muscular exertion, as in the case of hunted 

 animals or soldiers killed in battle, is usually followed quickly by 

 muscle rigor; indeed, in extreme cases it may develop almost imme- 

 diately. Death after wasting diseases is also followed by an early 

 4 



