SMOOTH MUSCLES 



447 



The following table, compiled by Blix, contains some data on the maximal 

 muscular capacity of man at different kinds of work : 



It is evident that the capacity for work calculated per second is greater, the 

 shorter the total time occupied. The highest record of endurance yet made was 

 observed in a six-day bicycle contest in New York. According to Atwater's cal- 

 culation the victor performed during his first day of twenty-three hours and 

 ten minutes an average of 25 kg. m. per second, and in the whole time, one 

 hundred and eight hours and forty-four minutes, an average of 20.2 kg. m. per 

 second. 



7. RIGOR MORTIS 



A muscle cut out of the body or excluded from the circulation passes 

 sooner or later (ten minutes to several hours) into a rigid condition known 

 as the death stiffening or rigor mortis. It is now shorter, thicker and firmer, 

 turbid, opaque and less extensible ; its reaction is acid, probably owing to the 

 transformation of a portion of the diphosphates into monophosphates brought 

 about by the lactic acid formed. 



Rigor of muscle is produced also by warming to 48-50 C., by the effects 

 of distilled water, by acids and by various other substances. It appears more 

 readily after heavy muscular work than otherwise, but on the other hand appears 

 later if the muscle has been paralyzed by section of its nerve. Rigor mortis is 

 the cause of the rigidity of the body after death. Under certain circumstances, 

 which have not yet been successfully imitated, the stiffening comes on immedi- 

 ately after death, so that the body becomes fixed in the position it had at the 

 instant of death. 



Rigor is regarded by most authors as coagulation of the muscle proteids. 

 But the processes taking place in rigor are only partially explained in this 

 way, for we do not even know definitely how the proteids obtainable from 

 muscle plasma occur in the living muscle. Besides, the phenomena of coagu- 

 lation in a muscle extract, and the rigidity brought about artificially by dif- 

 ferent reagents, present several points of difference from the natural death 

 stiffening. 



8. SMOOTH MUSCLES 



The most satisfactory smooth muscles for study are those whose fibers run 

 parallel, like the retractor penis of the dog and the circular muscles in the stom- 

 ach of the frog. The extensibility of such muscles is relatively great and the 

 elastic after-effect is very considerable. A small weight acting for a long time 



