SKELETAL MUSCLE 73 



the muscle does not have time to relax, so a series of 

 stimuli can be 'given at so rapid a rate that the muscle 

 is held throughout at the height of contraction. Shocks 

 sent in with a frequency of 25 in a second should suffice 

 to secure this result. The tracing obtained from a 

 muscle thus treated has a plateau character and the 

 temptation is to describe such a contraction as con- 

 tinuous. But it is not to be thought of in this way. 

 The seeming continuity of the process is the result of 

 successive physiologic acts of contraction permitting no 

 apparent relaxation to take place. Sustained con- 

 tractions are common enough in our experience; in fact, 

 they are the rule rather than the exception. Even our 

 briefer voluntary movements are believed to be produced 

 by stimuli which issue in series rather than singly from 

 the nervous system. 



A tetanic contraction is, therefore, the response of a 

 muscle to rapidly repeated stimuli and is contrasted 

 with the single contraction which is the response to a 

 solitary stimulus. The name tetanus, in medical litera- 

 ture, means lock-jaw, and since in that disease the chief 

 symptom is the recurrence of muscular spasms the term 

 used by the physiologist is seen to be appropriate. A 

 tetanus may be either complete or incomplete. In 

 the second case relaxation begins after each upthrust but 

 is interrupted the next instant. A jagged record results 

 with the experimental preparation and the contraction 

 is obviously of a tremulous or fluttering character. 

 Strong voluntary contractions are attended by notice- 

 able tremor. This has been attributed to incomplete- 

 ness of tetanus in the fibers at work, but it may also be 

 due to varying distribution of activity among different 

 portions of the muscles. 



Muscular Fatigue. If a frog's muscle is compelled 

 by stimulation to make a long series of simple contrac- 

 tions, several points of interest will apear in the record. 

 It often happens that the first few contractions are 

 seen to have gained progressively in height. This 



