THE MUSCULAR SYSTEM 



419 



muscle ; the appearance presented in this variety of muscle may 

 be due to cold, for it has been shown that two or three days after 

 death smooth muscle may be warmed up so as to be capable of 

 contraction. 



Phenomena of Contraction in Smooth Muscle.— Though there 

 is a marked difference in appearance between red and smooth 

 muscle, the actual phenomena of contraction do not differ 

 excepting in the matter of rate. The latent period, contraction, 

 and relaxation, are present, as in red muscle, but occur more 

 slowly ; they are, in fact, sluggish and deliberate. Owing to the 

 existence of numerous nerve fibres and ganglia, smooth muscle 

 may be completely isolated from all nervous connections, and 

 still continue to exhibit the phenomena of muscular contraction. 



In response to a continuous or induced current, smooth muscle 

 behaves much as does red, excepting, of course, that the response 

 is slower — as much as from 100 to 500 times slower. Summation 

 also is present, though not identical with that observed in red 

 muscle, for no contraction follows the first three or four stimuli ; it 

 is the stimuli which here accumulate before contraction follows, 

 and after this has occurred the muscle subsequently responds to 

 further stimuli by an increased height of contraction as does a 

 red muscle. 



Mechanical stimulation of smooth muscle excites a sluggish but 

 marked response ; pinching the intestines produces peristalsis, 

 and even drawing the finger lightly over the stomach wall may 

 produce ' weals ' of contraction. The muscle is markedly 

 responsive to tension, resembling in this a skeletal muscle which, 

 as we have seen, up to a certain ' load ' does more work the 

 greater the weight it has to lift. In the case of the smooth 

 muscle of the digestive canal the chief source of stimulation in the 

 herbivora lies in the cellulose and indigestible fibres in the food, 

 the bulkiness of their diet providing the mechanical stimulation 

 and distension necessary to produce the tension to which this 

 muscle is so responsive. The same remark may also apply to the 

 physiological action of the stomach and intestinal gases ; nor 

 must the bladder be omitted from this consideration, from the 

 point of view that distension by fluid provides a tension of the 

 walls which acts as a stimulus to contraction. There are two 

 distinctive physiological features associated with smooth muscle, 

 one is its capacity for remaining contracted for a long period, 

 and the other is its rhythmical activity. 



