110 



THE PHYSIOLOGY OF MUSCLE AND NERVE. 



perimental purposes, the arrangement being as represented in 

 Fig. 49. 



When the conductivity of the nerve is interrupted by any of the methods 

 described above, certain 'peculiar reactions may be obtained in the inter- 

 mediate stages before conduction is entirely abolished. The most interest- 

 ing of the stages is the paradoxical condition. In this stage a weak stimulus 

 applied at a will cause a contraction of the muscle, while a stronger stimulus 

 will prove ineffective. Wedenski,* who has studied these reactions with 

 great care, believes that the nerve in the narcotized area is thrown into a 

 peculiar condition of continued excitation to which he gives the name of 

 parabiosis. The condition is supposed to be characterized physiologically 

 by a loss of lability of the living material. It seems possible, however, that 

 the reactions which are taken as characteristic of the parabiotic condition 

 may be explained upon the assumption that the narcotics and other reagents 

 mentioned so alter the nerve as to make it more susceptible to fatigue (see 

 following paragraph). 



The Question of Fatigue of Nerve Fibers. An important 

 question in connection with the nature of the nerve impulse has 



been that of the suscep- 

 tibility of the nerve fibers 

 to fatigue. The obvious 

 fatigue of muscles and 

 of nerve centers has been 

 referred to the accumula- 

 tion of the products of 

 metabolism of their tis- 

 sues or to the actual 



consumption of the en- 



-IT , 



ergy-yielding material in 

 ,1 * -1-1 , i , 

 them, functional actlV- 



ityin these tissues im- 



Fig. 49. Schema to show the method of block- 

 ing the nerve impulse by means of a polarizing cur- 

 rent: a, The stimulating electrodes; 6, the battery, 

 the current of which is led into the nerve. The de- 

 pressed irritability at both anode, + , and cathode, , 



re g 



plies the breaking down 



of complex organic material (catabolism) and the setting free 

 of the so-called chemical energy. The potential, chemical, or internal 

 energy of the compound is liberated as kinetic energy of heat, etc. 

 It has been accepted, therefore, that, if the nerve fiber could be 

 demonstrated to show fatigue as a result of functional activity, 

 this fact would be probable proof that the conduction of the im- 

 pulse is associated with a chemical change of a catabolic nature in 

 the substance of the fiber. Experimental work, however, has 

 shown that under normal conditions the nerve fiber shows no 

 fatigue. The experiments made upon this point have been nu- 

 merous and varied. The general idea underlying all of them has 

 been to stimulate the nerve continuously, but to interpose a block 

 somewhere along the course of the nerve so that the impulses should 

 not reach the end-organ. This precaution is necessary because 

 * Wedenski, "Pfliiger's Archiv," 100, 1, 1904. 



