CH. XIV.] CONTRACTION OF INVOLUNTARY MUSCLE 159 



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 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 in the heart 

 elicits a maximum contraction (" all or nothing " Waller) ; the pheno- 

 menon known as the staircase (see p. 117) 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. It does not obey the " all or nothing " law. 



The normal contraction of voluntary muscle is a kind of tetanus 

 (see p. 121) ; 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. 145). 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 this electrical change is examined with the electrometer, 

 it is seen that it is a diphasic one as in voluntary muscle ; 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 168) show this fact graphically. 



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. Bayliss and Starling describe 

 the ventricular contraction of the mammalian heart as being accompanied by a 

 triphasic electrical variation ; this is due to the contraction at the base outlasting 

 that at the apex ; if, therefore, base and apex are led off to the electrometer, the 

 first phase is due to increased positivity at the base, the second to that at the apex ; 

 this quickly subsides, but the increased positivity at the base which still continues 

 accounts for the third excursion of the mercury. 



