244 



PRACTICAL PHYSIOLOGY. 



[XLV1II. 



(A.) Make a nerve-muscle preparation, attach a straw flag to the 

 foot, and fix the femur in a clamp, as in fig. 1 68. Lay the nerve 

 over the electrodes. Trace the direction of the current, and make 

 a mark to guide you as to when the current in the nerve is 

 descending or ascending, *>., whether the negative or positive pole 

 is next the muscle. 



((.) Place a drop of a saturated solution of common salt on the 

 nerve between the electrodes and the muscle. In a minute or less 



FIG. 168. Scheme of Electrotonic Variation of Excitability. D. Drop of strong solution 

 of salt on the nerve, N ; F. Flag on the muscle. 



the toes begin to twitch, and by-and-by the muscles of the leg 

 become tetanic, so that the flag is raised and kept in the horizontal 

 position. 



('/.) Turn the commutator, so that the positive pole is next the 

 muscle ; the straw sinks, i.e., the excitability of the nerve in the 

 region of the positive pole is so diminished as to " block " the 

 impulse passing to the muscle, showing that the positive pole 

 lowors the excitability. - 



FlQ. 169. Scheme of Electrotonic Variation of Excitability. P, P. Polarising, 

 and E, E. Stimulation current. 



(<".) Reverse the commutator, so that the negative pole is next 

 the muscle. The limb becomes tetanic, the negative pole 

 (katlielectrotonic area] increases the excitability. 



2. Another Method. Apparatus. Three Daniell's cells, two pairs of N.P. 

 electrodes, two Du Bois keys, a spring-key, commutator with cross-bars, 

 induction coil, wires, moist chamber, drum. 



B. (a,) Arrange the apparatus according to the scheme (fig. 169). Prepare 

 two pairs ol N.P. electrodes lor the nerve. 



