568 THE LAW OF CONTRACTION. 



was a descending one, then this tetanus ceases at once after section of the intrapolar area, a 

 proof that the tetanus resulted from the now separated anode. If the current was an ascending 

 one, section of the nerve has no effect on the tetanus. 



Pfluger and v. Bezold found a further proof that the closing or make contraction proceeds from 

 the cathode, and the opening or break contraction from the anode, by showing that with a descend- 

 ing curreiut, the closing contraction in the muscle, at the moment of closing occurred earlier, 

 while the opening contraction at the moment of opening occurred later; and, conversely, with 

 an ascending current the closing contraction occurred later, and the opening contraction 

 sooner. The difference in time corresponds to the time required for the propagation of the 

 impulse in the intrapolar region ( 337). If a large part of the intrapolar region in a frog's 

 nerve be rendered inexcitable by applying ammonia to it, then only the electrode next the 

 muscle stimulates, i.e., always on closing or making a descending current and on opening or 

 breaking an ascending one (Bicdcrmann). 



A. The law of contraction is valid for all kinds of nerves I. The contraction 

 occurring at the closing or opening of a constant current varies with (a) the direc- 

 tion (Pfaf)y and (6) the strength of the current {Heidenhain). 



(1) Very feeble currents, in conformity with the third of the above statements, 

 cause only a closing contraction, both with an ascending and a descending current. 

 The disappearance of electrotonus is so feeble a stimulus as not to excite the nerve. 



(2) Medium currents cause opening or closing contractions both with an ascending 

 and descending current. 



(3) Very strong currents cause only a closing contraction with a descending 

 current ; the opening shock does not occur, because, with very strong currents, 

 almost the whole of the intrapolar portion of the electrotonic nerve is incapable of 

 conducting an impulse (p. 565). Ascending currents cause only an opening contraction 

 for the same reason. With a certain strength of current, the muscle remains 

 tetanic while the current is closed (" closing tetanus"). 



[The lav of contraction is formulated : B = rest ; C = contraction.] 



II. In a dying nerve, losing its excitability, according to the Bitter- Valli law 

 ( 325, 7), the law of contraction is modified. In the stage of increased excitability, 

 weak currents cause only closing contractions with both directions of the current. 

 In the following stage, when the excitability begins to diminish, weak currents cause 

 opening and closing contractions with both currents. Lastly, when the excitability 

 is very greatly diminished, the descending current is followed only by a closing 

 contraction, and the ascending by an openiug contraction (Ritter, 1829). 



III. As the various changes in excitability occur in a centrifugal direction along 

 the nerve, we may detect the various stages simultaneously at different parts along 

 the course of the nerve. According to Valentin and Tick, the living intact nerve 

 shows only a closing contraction with both directions of the current, and opening 

 contractions only with very strong currents. 



Fleischl's Law of Contraction. V. Fleischl and Strieker have stated a different law, in 

 respect to the fact, that the excitability varies at certain points in the course of a nerve. The 

 sciatic nerve is divided into three areas : (1) Stretches from the muscle to the place where the 

 branches for the thigh muscles are given off; (2) from here to the intervertebral ganglion ; (3) 

 from here into the spinal cord. Each of these three areas consists of two parts ("upper and 

 lower pole "), which adjoin each other at an equator. In each upper pole, the excitability of 

 the nerve is greater for descending currents, and in each lower pole for ascending ones. At 

 each equator the excitability of the nerve is the same for ascending and descending currents. 

 The difference in the activity, due to the direction of the current, is greater for each stretch of 

 nerve the greater this stretch is distant from the equator. The excitability is less at those 

 points of the nerve where the three areas join each other. 



