THE PHENOMENON OF CONTRACTION. 55 



from the direct control of the will, being regulated by reflex and 

 usually unconscious Sixmuiations from the central nervous system. 

 All the important movements of the internal organs, or, as they 

 are sometimes called, the organs of vegetative life, are effected 

 through the activity of this contractile tissue. From this stand- 

 point their function may be regarded as more important than that 

 of the mass of the voluntary musculature, since so far as the mere 

 maintenance of the life of the organism is concerned, the proper 

 action and co-ordination of the movements of the visceral organs 

 is at all times essential. 



Distinctive Properties. — The phenomena of contraction shown 

 b}^ 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 



Fig. 26.-;-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 oi stimulation and beginning cf contraction is equal to about three 

 seconds. 



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

 ing generally, the simple contractions of plain muscle have a ver\' 

 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 



