GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 105 



of muscles is slight but quite perfect, by which is meant that a muscle yields 

 readily to a stretching force, but on the removal of the force quickly recovers 

 its normal form. Most of the experiments upon muscle elasticity have been 

 made after the muscle had been removed from the body, hence under abnormal 

 conditions. Under these circumstances it is found that if a number of equal 

 weights be added to a suspended muscle, one after the other, the extension pro- 

 duced is not, like that of an inorganic body 

 such as steel spring, proportional to the weight, 

 but each weight stretches the muscle less than 

 the preceding. If the weights be removed 

 in succession, an elastic recovery is observed, 

 which, although considerable, is incomplete. 

 If the change in the length be recorded by 

 a lever attached to the muscle, the surface 

 being moved along just the same amount after 

 each weight is added or removed, a curve is 

 obtained such as is shown in Fig. 37, 6. 

 Above this is a record taken in a similar way 

 from a piece of rubber (a). The rubber resem- 

 bles a steel spring in that equal weights stretch 

 it to like amounts, but the elastic recovery, 

 though more complete than that of the muscle, 

 is imperfect. 



In such an experiment it is found that the 

 full effect of adding the weights, or removing 

 them from the muscle, does not occur immedi- 

 ately, but when a weight is added there is a 

 gradual yielding to the stretching force, and, on the removal of a weight, a 

 gradual recovery of form under the influence of the elasticity. This slow 

 after-action makes it difficult to say just what is to be considered the proper 

 curve of elasticity of muscle, especially as the physiological condition of the 

 muscle is always changing. The elasticity of muscles is dependent on normal 

 physiological conditions, and is altered by death, or by anything which causes 

 a change in the normal constitution of the muscles, as the cutting off of the 

 blood-supply. The dead muscle is less extensible and less elastic than the 

 normal living muscle. Heating, within limits, increases, and cooling decreases 

 the elasticity. Contraction is accompanied by increased extensibility, i. e. 

 lessened elasticity, and the changes caused by fatigue lessen the elasticity. It 

 is interesting to note in this connection that the elasticity is decreased by weak 

 acid solutions and increased by weak alkaline solutions (Brunton and Cash). 1 



The elasticity of a muscle within the normal body is without doubt more 



perfect than that of an isolated muscle, and suffices to preserve the tension of 



the muscle under all ordinary conditions. The muscles are attached to the 



bones under elastic tension, as is shown by the separation of the ends in case 



1 Philosophical Transactions, 1884, p. 197. 



FIG. 37. o, Curve of extensibility 

 and elasticity of a rubber band ; b, curve 

 of extensibility and elasticity of a sar- 

 torius muscle of a frog. The weights 

 employed were 10 grams each. The 

 same length of time was allowed to 

 pass between the adding and subtract- 

 ing of the weights. 



