STIMULI AND THEIR ACTIONS 415 



latter, has been reduced to a very definite principle, and this 

 principle finds expression in the law of polar excitation. 



If a constant current be sent through a motor nerve, the nerve 

 becomes stimulated at the kathode upon making, and from here 

 the excitation extends through the conducting nerve to the muscle, 

 which latter then performs a contraction. Upon breaking the 

 current the excitation of the nerve takes place at the anode and 

 extends from here to the muscle, so that the latter contracts. In 

 the year 1859 Pfliiger confirmed this law of polar excitation 

 of the nerve. Its correctness can be proved in various ways, best 

 by the following experiment. A constant current is allowed to 

 flow in different directions through the nerve of a nerve-muscle 

 preparation, first, descending, i.e., with the anode lying nearer the 

 central end of the nerve, the kathode nearer the muscle, and, secondly, 

 ascending, i.e., with the anode lying nearer the muscle and the 

 kathode nearer the central end of the nerve ; and both times the 

 contraction of the muscle is recorded upon the plate of a myograph. 1 

 It is then found from the length of the latent period that at 

 the making of the descending current the muscle contracts earlier 

 than at the making of the ascending current, but that at the 

 breaking the relation is reversed ; the difference in time equals the 

 duration of the process of transmission of the stimulus along the 

 intrapolar stretch of nerve. It thus appears that the excitation 

 upon making must start from the kathode, and upon breaking from 

 the anode. This law of polar excitation was recognised by Bezold 

 ('61) as valid for cross-striated muscle, and Engelmann (70) 

 showed that it can also be applied to smooth muscle. Later 

 investigations, especially by Biedermann (79, '83, '84, '85), 

 afforded a number of new proofs of its validity. It was then 

 assumed that, like muscle and nerve, all living substance is 

 stimulated by the galvanic current at the kathode upon making,, 

 and at the anode upon breaking. But it has here been demon- 

 strated again how the one-sided investigation of nerve and muscle 

 may lead to errors, which can be avoided by comparative physio- 

 logical research, for the testing of other forms of living substance, 

 especially various kinds of free-living cells, has shown that a generally 

 applicable law of polar excitation of living substance does not 

 exist. Since the phenomena in question in unicellular organisms 

 afford a striking example of the fact that excitation is caused not 

 only by variations of current, but also by the continued current, we 

 will here consider them somewhat in detail. 



In the year 1864 Kiihne made the peculiar observation 

 that Actinosphcerium EichJiornii (Fig. 198) obeys a very different 

 law of excitation. But this discovery remained isolated and un- 

 noticed for more than two decades. Only when certain other 

 effects of the galvanic current, constituting galvanotaxis, were 

 1 Of. Fig. 154, p. 362. 



