NERVE-DEGENERATION AND NERVE-REGENERATION. 637 



be found that the nerves of animals poisoned with carbon dioxid, alcohol, 

 cocain, curare, or coniin, occasionally also the nerves in paralyzed parts 

 of the body in man, are no longer responsive to local stimuli, although 

 they still conduct impulses from the central areas. The injured seg- 

 ment of nerve thus loses its irritability earlier than its conductivity. 

 The analogous phenomenon in muscle-fibers is discussed on p. 114. 



After the administration of certain poisons to living animals, especi- 

 ally veratrin, the irritability of the nerves is at first increased, then 

 diminished to the point of complete abolition, as indicated by the extent 

 of the contractions in the muscle supplied by the affected motor nerve. 

 In the case of other poisons the abolition of irritability takes place rapidly, 

 as, for example, that induced by curare. Coniin, cynoglossum, methyl- 

 strychnin iodid, and ethyl-strychnin iodid. 



If a frog-preparation consisting of nerve and muscle be placed in a poisonous 

 solution results are occasionally manifested which are different from those pro- 

 duced if the poison is administered internally to the living animal. Atropin, 

 for example, gives rise to a reduction in the irritability of the preparation, with- 

 out preceding increase. Alcohol, ether, and chloroform, first increase and then 

 diminish the irritability. 



If the nerve is separated mechanically from its connection with its 

 centers, as by section, or if the center has undergone degeneration, the 

 nerve is first thrown into a state of increased irritability beginning in its 

 central extremity and extending toward the periphery ; then the irrita- 

 bility diminishes to the point of complete abolition. This process takes 

 place more rapidly within the portions of the nerve nearer the center 

 than in the more distal portions. This phenomenon is known as the 

 Ritter-Valli law. 



The rapidity of conduction of stimuli in nerves is increased in the stage of 

 increased irritability and diminished in that of lowered irritability. In the latter 

 stage the current, on electrical stimulation, must be continued for a longer time 

 in order to be effective, therefore the rapidly successive shocks of the induced 

 current are generally ineffective. Also the law of muscular contraction is modified 

 in the various stages of the alteration in irritability during the process of de- 

 generation. 



Finally, attention should be called to the fact that some nerves 

 possess a greater irritability at certain points, and that they retain this 

 slightly longer at such points. 



Thus, for example, the sciatic nerve of the frog in its upper third is more 

 irritable to various stimuli, in both its sensory and its motor fibers, than at a more 

 distal portion. Such inequalities in the irritability are due alone to accidental 

 injuries inflicted in the course of preparation. A branch is given off from the 

 upper third of the sciatic. According to Beck, in an uninjured nerve the more 

 central portions require stronger stimuli than the more peripheral to induce the 

 first minimal effect. 



After section or crushing of a nerve all of the electrical currents employed 

 for the stimulation of the nerve that pass in the nerve from the site of the lesion 

 exert a much more active influence than those in an opposite direction. The cause 

 for this resides in the fact that the current developing in the nerve after the injury 

 is added to the electrical stimulating current. Also in an uninjured nerve, for 

 example the sciatic of the frog, there are points at the central or peripheral ter- 

 mination of the nerve, or where large branches are given off, that react in a manner 

 similar to the sites of lesion previously mentioned. 



The dead nerve has lost its irritability completely. Death advances 

 in accordance with the Ritter-Valli law from the central organs of the 

 nervous system gradually to the peripheral paths. An acid reaction, 



