CH. XIII.] CONTRACTION OF INVOLUNTARY MUSCLE 141 



that portions of muscular tissue entirely free from nerves act in the 

 same way as those that possess nerves) indicates that it is the 

 muscular rather than the nervous tissues that possess these properties ; 

 though it can hardly be doubted that under normal conditions the 

 contraction of involuntary muscle is influenced and controlled by 

 nervous agency. 



As instances of nerveless involuntary muscles which possess the 

 property of rhythmical action, we may take the ventricle apex of the 

 frog's or tortoise's heart. If this is cut off and fed with a suitable 

 nutritive fluid at considerable pressure it will beat rhythmically 

 (Gaskell). The middle third of the ureter is another instance of 

 muscular tissue free from nerves, but which nevertheless executes 

 peristaltic movements. Perhaps, however, the most striking example 

 is that of the foatal heart, which begins to beat directly it is formed, 

 long before any nerves have grown into it. 



The artificial stimuli employed for involuntary are the same as 

 those used for voluntary muscle ; single induction shocks are, however, 

 often ineffectual to produce contraction, but the make, and to a less 

 extent the break, of a constant current will act as a stimulus. 



The faradic current is a good stimulus, but it never throws 

 involuntary muscle into tetanus; in the heart, strong stimulation 

 will sometimes effect a partial fusion of the beats, but never complete 

 tetanus. The rate of stimulation makes no difference ; in fact, very 

 often a rapid rate of stimulation calls forth less rapidly occurring 

 contractions than a slow rate. 



It is possible to render the frog's heart quiescent by tying a 

 ligature tightly around the junction of the sinus with the right 

 auricle, but the heart can be made to contract on stimulating it. 

 It is then found that the latent period is much longer, than in 

 voluntary muscle ; if a series of stimuli are applied, say, at intervals 

 of a second or two, each produces a single heart-beat ; the successive 

 contractions so obtained show a well-marked staircase (beneficial 

 effect of contraction, see p. 101). The strength of the stimulus in 

 such an experiment does not matter; a minimal stimulus elicits a 

 maximum effect (" all or nothing " Waller). 



The contraction of smooth muscle is so sluggish that the various 

 stages of latent period, shortening and relaxation, can be followed 

 with the eye; the latent period often exceeds half a second in 

 duration. It does not obey the " all or nothing " law. 



The normal contraction of voluntary muscle is a kind of tetanus 

 (see p, 104) ; the normal contraction of cardiac and plain muscle 

 is a much prolonged single contraction. A very valuable piece 

 of evidence in this direction is seen in the experiment on the heart 

 with the physiological rheoscope (see p. 128). Each time the 

 heart contracts the rheoscopic preparation executes a single twitch. 



