244 



PRACrriCAL PHYSIOLOGY. 



[xLvni. 



(b.) Make a nerve-miiycle preparation, attach a straw flag to the 

 foot, and fix the femur in a clamp, as in fig. i68. 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, i.e., whether the negative or positive pole 

 is next the muscle. 



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

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



Fia. i68.— 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. 



((L) 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 

 lowers the excitability. 



FlO. 169. — Scheme of Electrotonic Variation of Excitability. P, P. Polarising, 

 and JS, E. Stimulation current. 



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

 the muscle. The hmb becomes tetanic, the negative pole 

 {kathelectrotonic 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 of N.P. electrodes for the nerve. 



