52 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



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 

 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 

 rigor, which in this case is of a more feeble character and shorter 

 duration. The development of rigor is very much hastened by many 

 drugs that bring about the rapid death of the muscle substance, such 

 as veratrin, hydrocyanic acid, caffein, and chloroform. A frog's mus- 

 cle exposed to chloroform vapor goes into rigor at once and shortens 

 to a remarkable extent. Rigor is said also to occur more rapidly 

 in a muscle still connected with the central nervous system than 

 in one whose motor nerve has been severed. After a certain 

 interval, which also varies greatly, from one to six days in human 

 beings, the rigidity passes off, the muscles again become soft and 

 flexible; this phenomenon is known as the release from rigor. In 

 the cold-blooded animals the development of rigor is very much 

 slower than in warm-blooded animals. Upon an isolated frog's 

 muscle the most striking fact regarding rigor mortis is the shortening 



