52 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



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. 

 One observer, Bottazzi, for example, has attempted to show that 

 tonus contractions are referable to changes in the sarcoplasm of 

 the fiber, whereas the ordinary contraction or twitch is effected 

 through the striated fibrillse. 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 tempera- 

 ture 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, and 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 

 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 



