MUSCULAR CONTRACTION. 133 



in strychnine poisoning and hydrophobia, in which most of 

 the muscles of the Body are thrown into prolonged and 

 powerful involuntary contractions. 



If, while a frog's muscle is still shortening under the in- 

 fluence of one electric shock, another stimulus be given it, 

 it will contract again and the neAV contraction will be added 

 on to that already existing, without any period of elongation 

 occurring between them. While the muscle is still con- 

 tracting under the influence of the second stimulus a third 

 electric shock will make it contract more, and so on, until 

 the muscle is shortened as much as is possible to it for that 

 strength of stimulus. If now the stimuli be repeated at 

 the proper intervals, each new one will not produce any 

 further shortening, but, each acting on the muscle before 

 the effect of the last has begun to pass off, the muscle will 

 be kept in a state of permanent or " tetanic" contraction; 

 and this can be maintained, by continuing the stimuli, until 

 the organ begins to get exhausted or "fatigued" and it 

 then commences to elongate in spite of the stimulation. 

 When our muscles are stimulated in the Body, from the 

 nerve-centres through the nerves, they receive from the Lit- 

 ter about 20 stimuli in a second, and so are thrown into 

 tetanic contractions. In other words, not even in the 

 most rapid movements of the Body is a muscle made to 

 execute a simple muscular contraction; it is always a 

 longer or a shorter tetanus. When very quick movements 

 are executed, as in performing rapid passages on the piano, 

 the result is obtained by contracting two opposing muscles 

 and alternately strengthening and weakening a little the, 

 tetanus of each. 



Causes affecting the Degree of Muscular Contraction 

 Thc extent of shortening which can be called forth in a 

 muscle varies with the stimulus. In the first place, a 

 single stimulus can never cause a muscle to contract so 

 much as rapidly repeated stimuli of the same strength 

 since in the latter case we get, as already explained, several 

 simple contractions such as a single stimulus would call 

 forth, piled on the top of one another. With very power- 

 ful repeated electrical stimuli a muscle can be made to 



