J 16 niYSIOLOGY OF MUSCLES AND NERVES. 



in time and from the distance between the two irritated 

 points the rate of propagation of the excitement was 

 found to be 30 m. per second. The high figure as com- 

 pared with that found with the nerves of frogs is ex- 

 plained by the higher tem2:)erature of human nerves. 

 The rate of propagation would indeed be much lowered 

 if the temperature of the arm were considerably de- 

 creased by the use of ice. 



The above calculation of the rate of propagation is 

 made on the assumption that this rate is constant 

 throughout its duration. There is, however, nothing 

 to show that this is the case. On the contrary, it is 

 more probable that the propagation proceeds at first at 

 a greater and afterwards at a less speed. This may be 

 inferred from an experiment arranged by H. JMunk. If 

 three pairs of wires are applied to a long nerve, one 

 close to the muscle, another at the centre, and the 

 third considerably above, and then causing three con- 

 secutive curves to describe themselves on the myo- 

 graph plate by irritating these three points, it will 

 be found that the three curves are not equally removed 

 from each other ; on the contrary, the first and second 

 stand very near together, while the third is far from 

 the two former. More than double the time was re- 

 quired for the excitement to traverse the full distance 

 from the upper to the lower end than it took to traverse 

 the half-distance from the middle of the nerve to its 

 lower end. The simplest explanation which can be 

 given of this phenomenon is that the excitement during 

 its propagation is gradually retarded, just as a billiard ball 

 moves at first very quickly but afterward at a gradually 

 decreasing speed. The retardation of the billiard ball 

 is due to the friction of the underlj^ing surface. From 



