272 



PHYSIOLOGY 



presents in many cases rhythmic contractions; and can carry out a 

 peripheral adaptation to its environment. These rhythmic contrac- 

 tions are almost invariably observed if the muscular tissue be 

 subjected to a certain amount of tension, after separation from 

 the central nervous system. The rhythm of the contractions may 

 vary from one (spleen) to twelve (small intestine) contractions in the 

 minute. 



The stimuli for smooth muscle are essentially the same as for 

 striated. As we should expect, however, from the sluggish response 

 of this kind of contractile tissue, the optimum rate of change of 

 current which excites is very much slower than in the case of striated 

 muscle. Thus in many instances a single induction shock, even if 

 very strong, is powerless to excite contraction, and the make-induction 



FIG. 95. At the cathode K there is a small line of constriction, surrounded 

 by an area of relaxation. At the anode itself the muscle is relaxed, but 

 is strongly contracted on each side of the anode, so that on rough obser- 

 vation it Avould be thought that contraction occurred at the anode itself. 



shock of long duration and low intensity is always more efficacious 

 than the short sharp break- induction current. A still better stimulus 

 is the make or break of a constant current. When the latter form of 

 stimulation is used, response occurs at the make sooner than at the 

 break, and, just as in voluntary muscle, the make excitation starts 

 from the cathode and the break excitation from the anode. 



An apparent exception to this statement is afforded by the behaviour of 

 certain forms of involuntary muscle. In the intestine, in the skin of worms, 

 and in many other muscular tubes the smooth muscle-fibres are arranged in 

 two definite sheets, one consisting of longitudinal, the other of circular fibres. 

 If non-polarisable electrodes, connected with a constant source of current, be 

 applied to the surface of the small intestine, when the current is made there 

 will be apparently a strong contraction of the circular coat at the anode, which 

 spreads up and down the intestine, and a linear contraction of the longitudinal 

 coat at the cathode. The same result is observed in the earthworm and leech. 

 But careful observation shows in each case that the irregularity is really only 

 apparent, and that in the immediate neighbourhood of the anode there is 

 relaxation of both coats, with a contraction of the circular coat on each side, 

 and that at the cathode there is a contraction of both coats. The accom- 

 panying diagram (Fig. 95) will servo to show the condition of the circular coat 

 at each electrode. 



As a matter of fact, in consequence of the arrangement of the fibres, we have in 



