30 



EXPERIMENTAL PHYSIOLOGY 



CHAPTEE III 



A SINGLE CONTRACTION OF A FROG S MUSCLE. ITS MODIFICATION 

 UNDER CHANGES IN THE EXTERNAL CONDITIONS 



If a single stimulus of very short duration be applied to a muscle 

 or its nerve, the muscle responds by giving a contraction of very short 

 duration. This is termed a simple twitch, and is to be studied as in 

 the following experiment. 



Experiment 1. — Record a simple twitch of a muscle setting up the 

 apparatus in the following way (see fig. 37). Connect one terminal of a 

 battery b to one terminal of the primary coil p c, and the second terminal of 

 this to the mercury key k, and thence to the special break key k 1 , from the 

 second terminal of which a wire is connected to the battery. The details of 

 construction of the break key are shown in fig. 36. A brass pillar d rotates 



about a vertical axis upon two bearings, 

 and to it is fixed a bent brass rod a. 

 To the metal bearings of d a binding 

 screw b 1 is connected, and this is fixed 

 in an insulating base of vulcanite. A 

 brass tongue b slightly curved upwards 

 at its free end, is also fixed to the vul- 

 canite base, and is connected to the 

 second binding screw B 2 . The free 

 extremity of b is slightly notched to 

 receive the rod a, and the two are kept 

 firmky in contact by a screw, part of 

 which is seen in the figure under the 

 vulcanite base, which tends to force b 

 upwards. If a current be made to 

 enter at b 1 it will pass to d, thence along a to b, and so out from b-. If now a be 

 knocked on one side the current is broken directly a and b are separated. The 

 drum having been covered and smoked is placed in position, and the arm a 

 (fig. 37), fixed to a collar fitting on the axle of the cylinder, is brought into such 

 a position that there is a well-blackened smooth piece of paper at the front of the 

 drum, when the arm a touches the rod of the break key k 1 . The two terminals 

 of the secondary coil sc are connected to the two blocks of the Du Bois friction 

 key k 2 , and to the remaining two terminals are connected the two wires of the elec- 

 trodes E. A gastrocnemius-sciatic preparation is now excised, the femur fixed 

 to the cork plate of the myograph, and a fine thread tied to the tendon of the 

 muscle, thus connecting it to the vertical arm of the crank lever. A small weight 

 w is attached to the horizontal arm at a point near its axis, and the muscle so 

 fixed that the writing lever is horizontal or points slightly downwards. The 

 nerve is now laid across the wire electrodes, the key k- being kept closed. A 

 timing-fork f, or a chronograph, is arranged to write its tracing vertically 



Fig. 36.— A Break Key. 



