56 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



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. 

 26). 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 

 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- 



Fig. 26.-7-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. 



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 (neurogenic tonus), but the muscle, when completely 

 isolated from the 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 



