THE EXCITATION OF NERVE FIBRES 



, LAW OF CONTRACTION 



267 



c = contraction. C = strong contraction. T = tetanus. O = no effect. 



With the weakest currents excitation occurs only at make, since a make-stimulus, 

 i.e. the rise of catelectrotonus, is always more effectual than a break-stimulus, i.e. the 

 disappearance of anelectrotonus. With currents of moderate strength excitation 

 occurs both at make and break, being better marked at make, especially in the case of 



FIG. 116. Arrangement of experiment to demonstrate Pfliiger's law of contraction. 



descending currents. With very strong currents we get a contraction at make only 

 when the current is descending, since, when the current is ascending, the excitation 

 started at the cathode cannot pass the block at the anode. For the same reason a 

 break contraction is obtained only with an ascending current, since at the break of 

 a descending current there is a swingback of the nerve at the cathode to a condition 

 of diminished irritability, which effectually blocks the excitation started higher up 

 the nerve at the anode. 



The arrangement of the experiment for demonstrating Pfliiger's law is shown in 

 Fig. 116. The strength of the current is graduated by means of the rheochord, the 

 current being led into the nerve by means of non-polarisable electrodes. It is extremely 

 important in these experiments to avoid any injury or drying of the nerves at either of 

 the two electrodes, since the excitatory effect neither at make or break would be abolished 

 by local injury. 



These results, worked out chiefly on motor nerves, have been con- 

 firmed as far as possible experimentally on sensory nerve, and on muscle 

 and contractile tissues generally, and probably hold good for all irritable 

 living tissues. 



It is said that an anelectrotonus takes some time to attain its full height, 

 and a catelectrotonus reaches its maximum almost directly after the current 

 is made, and that it is on this account that a current of very short duration 

 excites only at the make, the break occurring before the anelectrotonus is 

 developed enough for its disappearance to cause a stimulus. 



Other things being equal, a current of given strength causes a stronger 



