196 



COMPARATIVE PHYSIOLOGY. 



holds equally well for nerve ; and the latter, though apparently 

 wholly different in structure at first sight, is really not so. 

 Nerve has its protoplasmic part (axis-cylinder), which is the 

 essential structure, its protective sheaths, and its nuclei (nerve- 

 corpuscles). 



As already indicated, a nerve possesses irritability. 



It is found that when the constant (polariziag) current is 

 passing from above downward — that is, when the cathode 

 (negative-pole) is on the side toward the muscle — the irritability 

 of the nerve is increased, and the reverse when the opposite 

 conditions prevail. 



This altered condition is known as electrotonus. 



It has been found as the result of many experiments that 

 profound modifications of the irritability of a nerve do take 

 place during the passage of a constant current. These are 

 diagrammatically represented in Fig. 181. 



Fig. 181.— Biagrammatic representation of variations in electrotonus according to 

 strength of current employed (after PflUger). nn', a section of nerve; a, anode 

 (+ pole); k, cathode (—pole). Curves above the horizontal denote catelectroto- 

 nus; below, the opposite. 



Briefly stated, they are these : 1. The nature of the change 

 depends on the direction of the polarizing (constant) current ; 

 hence, if the current is descending, there is an increase of irri- 

 tability (catelectrotonus) in the portion of the nerve nearest the 

 muscle, and vice versa. 2. The extent of the change of irrita- 

 bility is dependent on the strength of the polarizing current. 

 3. This change is most marked close to the electrodes, spreads 

 to a considerable extent beyond this point without the elec- 

 trodes (extra-polar regions), and also exists within the region 

 of contact of the electrodes (intra-polar regions). 4. It follows 

 that there must be a point at which it is not experienced (indif- 

 ferent point or neutral point). 



Now, it is possible to understand why a sudden change ia 



