244 



PHYSIOLOGY 



separation from the central nervous system. The rhythm of the contrac- 

 tions 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 in- 

 stances a single induction shock, even if very strong, is powerless to excite 

 contraction, and the make-induction 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 the 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 

 different sheets, one consisting of longi- 

 tudinal, 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 con- 



FIG. 94. At the cathode K there is a small line traction of the circular coat at the 

 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 observation the longitudinal coat at the cathode, 

 it would be thought that contraction occurred The same result is observed in the 

 at the anode itself. 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 accompanying 

 diagram (Fig. 94) will serve to show the condition of the circular coat at each electrode. 



As a matter of fact, in con- 

 sequence of the arrangement of 

 the fibres, we have in the neigh- 

 bourhood of the anode a num- 

 ber of places (virtual cathodes) 

 where the current is leaving the 

 muscle-cells to enter inert con- 

 ducting tissues, and in the same 

 way there will be in the neigh- 

 bourhood of the cathode a num- 

 ber of virtual anodes (Fig. 95). 



anode, which spreads up and down the 

 intestine, and a linear contraction of 





FIG. 95. Diagram to show the spread of current which 

 occurs when a current is led through a tube such as the 

 ureter by means of two electrodes applied to its surface. 

 It will be noticed that while + E is the anode, there are 

 immediately below and around it a number of cathodes, 

 E,, E /y , E,,,, E ///y due to the current leaving the muscle to 

 flow tlirough indifferent tissues. (BIEDERMANN.) 



Thus if we take the ureter and 



lead a current through it while 



it is slung up in thread loops 



serving as electrodes, there is contraction of both coats at the cathode and relaxation 



of both at the anode. If however the ureter be packed in a pulp of blotting-paper 



