STIMULATION OF MUSCLES AND OF NERVES 423 



" time stimuli," as they are called, is that the contractions which they induce 

 continue for a noticeably longer time, and the nerves and muscles can there- 

 fore- be thrown by them into a state of excitation which lasts longer, than is the 

 case with the sudden stimuli. This peculiarity, as we shall see later, is of great 

 importance for the theoretical explanation of voluntary muscular contraction. 



The nature of the irritable tissue again has much to do with the length 

 of time required to stimulate it: the more slowly it reacts, the longer must 

 the stimulus act to produce a visible effect. 



A single induction shock, which is so effective for the nerve, is but slightly 

 effective for smooth muscles. In certain stages of degeneration the skeletal 

 muscles exhibit a very much reduced or even absolute lack of sensitivity toward 

 induction currents, whereas their excitability to constant currents remains unim- 

 paired or may even be increased (Erb). With the rapidly contracting frog's 

 muscles, stimulation of the nerve by the break-induction shock is much stronger 

 than by the make-induction shock, because the former is a more sudden stimulus. 

 Nerve-muscle preparations of the turtle, which are much more sluggish in their 

 action, behave in exactly the reverse manner. 



3. Law of Contraction. The stimulating effect of the constant current 

 depends not only upon the strength but also upon the direction of the cur- 

 rent in the nerve. If the current is weak it generally produces a contraction 

 only when it is closed, no difference in which direction it is flowing. If the 

 current is increased in strength (medium current), contractions occur also 

 when it is opened, whether the current is flowing toward the muscle (de- 

 scending) or away from it (ascending), although opening contractions do 

 not always appear with the same strength of current in the two cases. In- 

 creasing the strength still more (strong current) we find that with the 

 ascending current the closing contraction gradually becomes smaller until it 

 finally disappears, while the opening contraction continues at its maximum. 

 With the descending current we find the reverse condition: the closing con- 

 traction remains at its maximum however strong the current be made, but 

 the opening contraction becomes smaller and smaller as the strength increases, 

 and not infrequently it disappears. However, the strength at which the clos- 

 ing contraction disappears, when the current is ascending, is not always the 

 same. And sometime.s when the current is descending the opening contrac- 

 tion does not disappear at all but persists at a certain minimal size. 



These generalizations may be summarized in the following formula, known 

 as Pfliiger's law of contraction : 



Strength of Current. Ascending current Descending current. 



,, r , ( Closing C C 



Weak . . \ ,, . ~ 



( Opening O 



,. j. ( Closing C C 



Medium { . ~ ~ 



Opening C C 



O C 



C O or weak C 



C = contraction ; no contraction. 



