IS2 MUSCLE AND NERVE 



KOC. When the stimulating current is increased gradually 

 in strength, they appear in the following order: KCC, ACC, 

 AOC, and KOC. This order of appearance is explained by 

 three facts: 



1. When the current is closed, the impulse is generated at 

 the kathode; and when it is opened, at the physiological anode. 



2. The impulse developed at the kathode is more effective 

 than the one developed at the anode. 



3. The effect of the current is greatest where the density is 

 greatest. 



When nerve and muscle are diseased, the ACC and KOC 

 are obtained respectively with weaker currents than KCC and 

 AOC. This is known as the reaction of degeneration. It has 

 been found that when a current is passed through a nerve or 

 muscle at right angles to the direction of its fibers it has no 

 stimulating effect. 



The irritability of a preparation depends 'upon changes in 

 its environment and upon changes within itself. Changes of 

 environment include mechanical agencies, temperature, chem- 

 ical agencies, and electrical currents. Mechanical agencies 

 acting on living substance usually first increase, but later 

 destroy the irritability. In general, cold decreases, while heat 

 increases the irritability, but in this case it is necessary to take 

 into consideration the character of the stimulus employed. 

 Tissues differ normally in their response to various stimuli. 

 A medullated nerve, for instance, responds better to an induced 

 current than to mechanical or chemical stimuli, but the appli- 

 cation of cold makes it more susceptible to the effects of mechan- 

 ical stimulation than to the exceedingly short induced current. 

 Chemicals first raise and then lower the irritability. Depriva- 

 tion of the blood supply is equivalent to a change in the chemical 

 environment of a tissue. That the normal irritability is depen- 

 dent upon the blood supply is shown by Stenson's experiment 

 on the rabbit, in which the abdominal aorta was closed by com- 

 pression. The paralysis which follows this procedure is due 

 successively to the loss of function of nerve cells in the cord, 

 then of the motor end plates, and finally of muscle and nerve 

 fibers. 



That the irritability of a preparation depends upon changes 

 that may take place within itself is shown by the separation 



