SMOOTH MUSCLE. 43 



with plain muscle, strong currents being required. Induction 

 shocks are less effective than galvanic. 



Mechanical stimulation in the form of stretching is by far the 

 most adequate form of stimulus, at least among artificial stimuli. 



Contraction. The LATENT PERIOD for smooth muscle response 

 may be as much as one hundred to five hundred times that of 

 striated muscle. In the frog's stomach it is from one to ten seconds; 

 for the cat's bladder, 0.25 sec., and for vascular muscles, 0.3 to 

 0.5 sec. 



The CONTRACTION PERIOD may be as much as 15 to 20 seconds 

 for the frog's stomach. The amount and duration of the con- 

 traction depends upon the stimulus. In the frog's stomach a 

 single contraction may decrease the stomach by 45%, while tetanus 

 may reduce it 59%. Although the contraction may be large, it is 

 much gentler than that of voluntary muscle. 



RELAXATION PERIOD. -The time consumed in relaxation is much 

 longer than that for contraction. In the frog's stomach it is said 

 to be from 60 to 80 sec. 



Summation and Tetanus. Two successive stimuli properly 

 spaced will cause summation. A series of stimuli will cause tetanus. 

 To do this they need not be very frequent. A stimulus every five 

 seconds is sufficient in the frog's stomach. 



Tone. Smooth muscle possesses the power of remaining in a 

 condition of persistent shortening for long periods. Thus hollow 

 viscera can adapt themselves to their contents. Tone can be 

 varied in different ways. Cooling increases it, while heat decreases 

 it. The most important means of controlling tone seems to be 

 through the extrinsic nerve supply, e.g., stimulation of the vagus 

 increases stomach tone, cutting the vagus produces flabbiness in 

 the same organ . Throughout life the walls of the arteries resist a high 

 pressure, this resistance being controlled by the nervous system. 



Rhythmicity. Smooth muscle in certain parts of the body 

 is able to undergo rhythmical contraction. These are the alimen- 

 tary canal, ureter, bladder, spleen and blood vessels. In the sto- 

 mach and intestine rhythmical activity is induced by tension. The 

 rate of these contractions is somewhat as follows: stomach, 3 per 

 min., intestine 12 per min., spleen, 1 per min. 



In this connection it is important to make it clear that these 



