l62 VOLUNTARY AND INVOLUNTARY MUSCLE. [CH. xiv. 



that possess these properties ; though it can hardly be doubted 

 that under usual circumstances the contraction of involuntary 

 muscle is influenced and controlled by nervous agency. 



The artificial stimuli employed for smooth muscle are the same 

 as those used for striated muscle ; single induction shocks are 

 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. 



A stimulus strong enough to produce a contraction at all 

 usually elicits a maximum contraction, but the phenomenon 

 known as the staircase (see p. 124) is generally better marked 

 in the case of the heart than in that of voluntary muscle. 



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. 



The normal contraction of voluntary muscle is a kind of 

 tetanus (voluntary tetanus, see p. 128); the normal contraction 

 of plain muscle is a much prolonged single contraction. A very 

 valuable piece of evidence in this direction is seen in the experi- 

 ment on the heart with the physiological rheoscope (see p. 153). 

 Each time the heart contracts the rheoscopic preparation executes 

 a single twitch, not a tetanus. This is an indication that the 

 electrical change is a single one, and not a succession of changes 

 such as occurs in tetanus. 



When, however, this electrical change is examined with the 

 electrometer, it is seen that it really is a diphasic one ; but in a 

 slowly contracting tissue like the heart-muscle the two phases are 

 separated by a prolonged period of equipotentiality, and thus they 

 are rendered more distinct.* The illustrations already given 

 (figs. 167 and 1 6 8) show this fact graphically. 



But though involuntary muscle cannot be thrown into tetanus, 

 it has the property of entering into a condition of sustained 



* When the heart is beating sluggishly in the rheoscopic experiment above 

 referred to, the separation of the two phases of the electrical change will 

 sometimes cause two twitches in the muscle nerve preparation. 



