THE PHENOMENON OF CONTRACTION. 53 



Distinctive Properties. The phenomena of contraction shown 

 by plain muscles are, in general, closely similar to those already 

 studied for striated muscle, the one great difference being the 

 much greater sluggishness of the changes. Plain muscles differ 

 among themselves, of course, as do the striated muscles, but, speak- 

 ing generally, the simple contractions of plain muscle have a very 

 long latent period that may be a hundred or five hundred times 

 as long as that of cross-striated muscle, and the phases of shortening 

 and of relaxation are also similarly prolonged; so that the whole 

 movement of contraction is relatively slow and gentle (see Fig. 

 25). Plain muscle responds to artificial stimuli, but the electrical 

 current is obviously a less adequate that is, a less normal stimulus 

 for this tissue than for the striped muscle. The amount of current 



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 is equal to about three 

 seconds. 



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 



* 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. 



