180 EXPERIMENTAL PHYSIOLOGY. 



(b.) When the muscle contracts, it merely makes a vertical 

 scratch on the smoked glass plate of the myograph. Move 

 the plate with the hand a little distance after each con- 

 traction. A series of vertical lines is obtained, and a 

 straight line will join the apices of them all. 



3. Seat of Exhaustion in Nerve and Muscle. 



A. (a.) Arrange an induction machine for interrupted 

 shocks. Connect the secondary coil with a Pohl's commuta- 

 tor without cross-bars. 



(b.) Arrange a Darnell's cell connected to N.P. electrodes, 

 and short-circuited for a constant current the " polarising 

 current " (Lesson XXXVIII., 2). 



(c.) Dissect out two nerve-muscle preparations (A and B) 

 from a frog, clamp both femurs in one clamp, and attach 

 straw-flags of different colours to both legs (Fig. 58). Lay 

 both nerves over a pair of du Bois electrodes. Arrange 

 a glass shade, lined in part with moist blotting-paper to 

 cover them and keep them moist. 



(d.) Attach the electrode wires to two of the binding 

 screws of the commutator, turning the handle, so that the 

 current can be passed through both nerves when desired. 



(e.) Arrange the nerve of B on the N.P. electrodes placed 

 between the tetanising current and the muscle, so that the 

 - pole is next the muscle. Pass an interrupted current 

 through both nerves, A will become tetanic, while B remains 

 quiescent ; the impulse cannot pass because of the effect of 

 the " polarising current " producing electrotonus. 



(f.) Continue to stimulate until the tetanus in A ceases. 

 Break the polarising current, B becomes tetanic. 



As both nerves have been equally stimulated, both are equally 

 fatigued. As B becomes tetanic, the seat of the fatigue is not in 

 the nerve trunk. 



