18 PHYSIOLOGY CHAP. 



maximal effect of the fusion of the two contractions takes place 

 when the interval between the two shocks is about -fa of a second 

 (Sewall and others). 



Summation of contractions takes place not only with currents 

 of medium strength, but also with maximal or supra-maximal 

 stimulation. The maximal contraction of a muscle is therefore 

 not obtained with a single stimulation, however strong, but only 

 with repeated stimuli in rapid succession, owing to the summation 

 of excitation. The interpretation of this phenomenon will be 

 given later in speaking of Contracture. 



When a series of stimuli act upon a muscle in rapid succession, 

 it reaches the maximum degree of shortening, owing to summation 

 of the stimuli, and remains in the state of persistent contraction 

 known as tetanus so long as the stimuli act upon it. The minimal 



FIG. 12. Comparison of tetanus curves from a red (?) and pale (p) muscle of rabbit. (Kronecker 

 and Stirling.) At A both muscles were excited by ten induced shocks per second ; at Z> by 

 six shocks per second. 



stimulation frequency necessary to produce complete tetanus 

 varies in the muscles of different animals. As a rule it is less 

 in proportion as the active phase of the muscular contraction is 

 slower. 12-30 stimuli per second suffice for frog muscles, while 

 20-30 are required for those of mammals. The red muscles of the 

 rabbit, according to Kronecker and Stirling, may be almost com- 

 pletely tetanised with 10 stimuli per second, while the pale muscles 

 of the same animal are only thrown into tetanus with 20-30 

 stimuli per second. With 6 stimuli per second the pale muscles 

 exhibit a series of almost wholly isolated contractions, while 

 the same frequency throws the red muscles into a tremulous 

 contraction closely resembling tetanus (Fig. 12). 



All conditions that make muscular contraction slower, as 

 fatigue, fall of temperature, etc., diminish the stimulation 

 frequency necessary for complete tetanus. On the cessation of 

 the series of stimuli that induced tetanic contraction, the muscle 

 never resumes its original length, but remains a little shortened 

 in consequence of the fatigue and the abnormal changes which the 



