76 NERVES. 



of effect produced is the greater, the shorter the time occu- 

 pied in the variation. No effect is produced if the current 

 is transverse. (3.) If the current is of moderate intensity, 

 the excitation occurs at make and break whatever its 

 direction the make excitation starting from the cathode, 

 the break from the anode. (4.) If the current is strong, 

 the make excitation is suppressed when the current is from 

 the muscle ; the break excitation when it is towards the 

 muscle. (5.) If the current is weak, there is no excitation 

 excepting at make. The propositions 3, 4, and 5 constitute 

 the so-called " Law of contraction." (Pfliiger.) If the 

 current lasts long and is of great intensity, reversed 'after 

 effects manifest themselves on its cessation. Thus there is 

 increased excitability at the anode which may lead to 

 excitation and manifest itself in contraction (Ritter's 

 Tetanus). This contraction is increased by reclosing the 

 current in the opposite direction annulled by reclosing it 

 in the same direction. 



The above experimental facts constitute the basis of the doctrine of Elec- 

 trotonus. The contrast between the two opposite states (called Cathelec- 

 trotonus and Anelectrotonus) referred to in (i), is most easily observed in the 

 parts of the nerve which are immediately beyond the limits of the part through 

 which the current passes ; but it can also be studied in the intrapolar part. 

 Here it is found that the cathelectrotonic effect diminishes in extent, and that 

 the anelectrotonic increases as the current becomes stronger. The statement 

 (4) is satisfactorily explained on the ground that the propagation of the make 

 excitation which originates at the cathode, is hindered by the anelectrotonus 

 which exists at the anode, and that in like manner the break excitation is 

 interrupted in consequence of the after effect at the cathode. The fact re- 

 corded in (5) which occurs invariably, simply means that the cathodic excita- 

 tion is stronger than the anodic. 



Methods and Processes of Excitation. (i) A motor 

 nerve may be excited by the closing or opening of a vol- 

 taic current flowing along it ; (2) by any change in the 

 intensity of such a current ; (3) by the passage along it of 

 an induction current ; (4) by the passage of a succession 

 of induction currents in alternately opposite directions 

 (Faradization) ; (5) mechanically either by a single per- 



