GENERAL NER^E PHYSIOLOGY 219 



(c) Chemical influences* These maybe classified as: 



1 . Those which destroy irritability and conductivity with- 

 out previously stimulating, e.g. acids, ammonia. 



2. Those which first stimulate and then paralyze, e.g. 

 concentrated salt solution, glycerin. 



If a part of a motor nerve is acted upon by carbon 

 dioxide, that part of the nerve loses its irritability for elec- 

 trical stimulations, but does not lose its conductivity; for if 

 the stimulation is applied above the place acted upon by the 

 carbon dioxide, the impulse is still carried to the muscle. 

 Hence irritability and conductivity are to a certain extent 

 independent of each other. 



(d) Electrical influences. A constant current passed 

 longitudinally through a stretch of nerve of sufficient length 

 causes : 



1. When made, stimulation and increased irritability at 

 the kathode (place of exit of current) ; decreased irritability 

 and conduction at the anode (place of entrance). 



2. When broken, decreased irritability and conduction, 

 for a short time, at the kathode; stimulation and also in- 

 creased irritability at the anode, lasting for a short time. 

 Stimulation by the electrical current which, in a motor 

 nerve, is manifested by the contraction of the muscle, as a 

 rule takes place only at the moment of the making and the 

 breaking of the current (make and break contraction) ; more 

 rarely it stimulates for some time during its passage through 

 the nerve (make-tetanus) and for some time after the break- 

 ing (break-tetanus). Hence the extent of stimulation of a 

 nerve depends chiefly on the changes in the strength of the 

 current, not upon its absolute intensity. Changes in the 

 strength of the current are the more effective the more 

 rapidly they take place. For this reason, when the current 

 is not suddenly made or broken but is made and broken 

 gradually, no contraction results. The make contraction is 

 stronger than the break contraction, so that a feeble but still 

 active current produces a contraction only at the make. 



