52 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



mediated by the fibrils, while the tone contractions are due to a 

 reaction in the sarcoplasm lying between the fibrils. Another sug- 

 gestion (de Boer*) is that the ordinary contractions are caused by 

 stimuh brought to the muscle through the cerebrospinal fibers, 

 while the tonic contractions are due to impulses conveyed through 

 sympathetic fibers. A still further suggestion uniting these two 

 views (Langelaanj) holds that the cerebrospinal fibers connect in 

 the muscle with the fibrils while the sjaxipathetic fibers make their 

 terminations in the sarcoplasm. None of these views have been 

 sufficiently well established to command general acceptance. 

 The phenomenon of tonus is important for the muscles of the trunk 

 and the hmbs, particularly in the maintenance of posture. It is 

 important also in the case of the abdominal muscles in the main- 

 tenance of a certain pressure upon the abdominal contents. Under 

 abnormal conditions, ill-health, shock, etc., muscular tonus may 

 be absent or reduced in amount, and this loss of tone may be de- 

 tected in the limb muscles by the lack of resistance to passive move- 

 ments of the limb, that is, movements effected through another 

 person. In the plain muscles of the arteries, intestines, and other 

 viscera tonus is even more striking and important than in the case 

 of the skeletal muscles, and frequent reference will be made to it in 

 the chapters dealing with the functions of these organs. 



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 

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



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

 t Langelaan, "Brain," 38, 191, 1915. 



