582 A TEXTBOOK OF PHYSIOLOGY 



flowing in the nerve from the muscle, it is said to be ascending; if 

 towards the muscle, descending (Fig. 323). In other words, it depends 

 upon the position of the point of entry of the current (the anode), and 

 the point of exit (the kathode). When a current begins to flow at make,* 1 

 excitability and conductivity is diminished in the region of the anode 

 and increased in the neighbourhood of the kathode ; when the current 



A 



FIG. 323. A,- As "ASCENDING" CURRENT. B, A "DESCENDING" CTTRRENT. 



ceases to flow, the condition of depressed excitability and conductivity 

 at the anode gives way, swings back, it may be said, to a condition of 

 raised excitability and conductivity, thus affording a stimulus. Simi- 

 larly, at the kathode the condition of raised excitability and con- 

 ductivity swings back to a condition of lowered excitability and 

 conductivity. At make, therefore, the kathode is the. exciting electrode; 

 at break, the anode. 



With weak currents, only the more efficient stimuli excite. Tho sudden increase 

 when the current is made is more effective than the sudden "swing-back" at the 

 anode when the current is broken; therefore, with weak currents in either direction, 

 contractions occur only at make : 



Ascending. Descending. 



Make. Break. Make. Break. 



C. O. C. O. 



With medium currents in both directions, there occur contractions at make from 

 the kathode and at break from .the swing-back at the anode : 



Ascending. Descending. 



Make. Break. Make. Break. 



C. C. C. C. 



With "strong" currents, the effect is modified according to the direction of thu 

 current. When the current is ascending, the anode lies between the exciting kathode 

 and the muscle. Around the anode the excitability and conductivity are so depressed 

 that no impulse gets through to the muscle, which is therefore only stimulated from 

 the anode at break. This stimulation sometimes induces a tetanus, known as " Bitter's 

 tetanus." 



With a strong descending current, the kathode being next the muscle, the impulse 

 generated at the kathode at make is not blocked in any way, and causes a contraction. 

 At break, however, the depressed excitability and conductivity which supervenes in 

 the kathodic region is sufficient to block ths impulse arising from the anode at 

 break. We have, therefore : 



