THE PHENOMENON OF CONTRACTION. 29 



Effect of Strength of Stimulus upon the Simple Contraction. 



— The strength of electrical stimuli can be varied conveniently and 

 with great accuracy. When the stimulus is of such a strength as 

 to produce a just visible contraction it is spoken of as a minimal 

 stimulus and the resulting contraction as a minimal contraction. 

 Stimuli of less strength than the minimal are designated as sub- 

 minimal. If one increases gradually the intensity of the electrical 

 current used as a stimulus without altering its duration, beginning 

 with a stimulus sufficient to cause a minimal contraction, the result- 

 ing contractions increase proportionally up to a certain maximum 

 beyond which further increase of stimulus, other conditions remain- 

 ing the same, causes no greater extent of shortening. Contrac- 

 tions between the minimal and the maximal are designated as 

 submaximal. (See Fig. 9.) 



This phenomenon of submaximal contractions was formerly 

 interpreted as a graded response to graded stimuli, and it was as- 

 sumed that a similar result would be obtained from a single fiber if 

 it were possible to experunent with it. A weak stimulus would 

 give a weak contraction and a stronger stimulus a stronger con- 

 traction, up to the maximum. Some years ago Lucas* found that 

 when a small slip of muscle containing a few fibers is stimulated it 

 does not give a smoothly graded increase in contraction as the 

 stimulus is increased, but, on the contrary, exhibits abrupt jmnps 

 from one level to another. He suggested, therefore, tliat the indi- 

 vidual fibers follow the "all or none law" with which we are 

 familiar in the case of heart muscle; that is to say, a fiber when 

 stimulated gives a maximal contraction or none at all. The sub- 

 maximal contractions observed in the whole muscle are simply the 

 expression of the activity of a part of the fi.bers. The theoretical 

 minimal contraction would be the contraction of a single fiber in 

 the group composing a muscle, while the theoretical maximal con- 

 traction would be the combined effect of the contraction of all the 

 fibers. This point of view seems to have found favor with most of 

 the workers who have investigated the matter, f 



Effect of Temperature upon the Simple Contraction. — Varia- 

 tions in temperature affect both the extent and the duration of the 

 contraction. The relationship is, however, not a simple one in the 

 case of the frog's muscle upon which it has been studied most fre- 

 quently. If we pay attention to the extent of the contraction alone 

 it will be found that at a certain temperature, 0° C. or slightly be- 

 low, the muscle loses its irritability entirely. As its temperature is 

 raised a given stimulus, chosen of such a strength as to be maximal 



* Lucas, "Journal of Physiology," 1905, 33, 125, and 1909, 38, 113. 

 t Consult Pratt, "American Journal of Physiology," 1917, 43, 159, and 44, 

 517, and Eisenberger, ibid., 1917, 45, 44. 



