THE PHENOMENON OF CONTRACTION. 53 



necessary to make it contract is far greater. The amount of con- 

 traction varies with the strength of stimulus, that is, the tissue 

 gives submaximal and maximal contractions. Two successive 

 stimuli properly spaced will cause a larger or summated contraction, 

 and a series of stimuli will give a fused or tetanic contraction. The 

 rate of stimulation necessary to produce tetanus is, of course, much 

 slower than for cross-striped muscle. The stomach muscle of the 

 frog, for instance, requires only one stimulus at each five sec- 

 onds to cause tetanus.* A distinguishing and important charac- 

 teristic of the plain muscle is its power to remain in tone, that 

 is, to remain for long periods in a condition of greater or less con- 

 traction. Doubtless this tonic contraction under normal relations 

 is usually dependent upon stimulation received through the ner- 

 vous system, but the muscle when completely isolated from the 



Fig. 25. Curve of simple contraction of plain muscle. The middle line is the time 

 record, marking intervals of a second. The lowermost line indicates at the break the mo- 

 ment of stimulation (short-lasting, tetanizing current). It will be seen that the latent period 

 between beginning of stimulation and beginning of contraction was equal to about three 

 seconds. 



central nervous system, whether in or out of the body, continues 

 to exhibit the phenomenon of tone to a remarkable degree. In 

 most of the organs in which plain muscle occurs there are present 

 also numerous nerve cells, and it is therefore still a question as to 

 whether the tonic changes shown by this tissue depend upon a 

 property of the muscle itself or upon their intrinsic nerve cells. 

 Most observers adopt the former view. The importance of this 

 property of tone in the plain muscle tissues will be made fully 

 apparent in the descriptions of the physiology of the organs of cir- 

 culation and digestion. Plain muscle may exhibit also the phenome- 

 non of rhythmical activity, that is, under proper conditions it may 



*Schultz, "Zur Physiologie der langsgestreiften (glatten) Muskeln," 

 "Archiv f. Physiologie," suppl. volume, 1903, p. 1. See also Stewart, "Amer- 

 ican Journal of Physiology, 4, 185, 1900. 



