OTHER FORMS OF CONTRACTILE TISSUE 273 



the neighbourhood of the anode a number of places (virtual cathodes) where 

 the current is leaving the muscle- cells to enter inert conducting tissues, and in 

 the same way there will be in the neighbourhood of the cathode a number of 

 virtual anodes (Fig. 96). 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 moistened with normal saline 



FIG. 96. 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 elec- 

 trodes 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 /X , E,,,, E,,,, due to the current leaving the muscle to flow through 

 indifferent tissues. (BIEDERMANN.) 



thus allowing the current to leave the contractile tissues anywhere along the 

 ureter, we get the same aberrant results of stimulation as are obtained with the 

 intestine. 



SUMMATION. If two stimuli be sent into a voluntary muscle 

 within a short interval of time, there is a summation of effect, the 

 contraction due to the second stimulus being piled, so to speak, on 

 the top of the first contraction. That a maximal twitch is not as 

 high as a tetanus, the production of summation of many twitches, 

 is due to the fact that the relaxation processes of a muscle begin 

 before it has time to overcome the inertia of the mass moved, and 

 so accomplish its maximum shortening. If therefore we support 

 the muscle in any way, whether by screwing up the lever (after-load- 

 ing) or by sending in a previous stimulus, the contraction due to a 

 stimulus will be more pronounced, until the shortening of the muscle 

 attains that observed in tetanus. For the same reason the height of 

 a single twitch in relation to a tetanus of the same muscle increases as 

 we slow the contraction, until, with a prolongation such as is produced 

 by veratrin, there is no difference at all between the height of a 

 maximal single contraction and the height of a tetanus. 



These considerations would lead us to expect no trace of any 

 process analogous to summation of contraction in the slowly moving 

 smooth muscle. In the heart muscle this is the case, no increase in 

 the height of a contraction being produced by sending in one, two, 

 or more shocks in quick succession. When, however, we record the 

 contractions of a muscle, such as the retractor penis, which is more 

 closely under the control of the nervous system, and excite with a 

 series of induction shocks, we get results which at first sight are 



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