T90 PRACTICAL PHYSIOLOGY. [XXXIIL 



LESSON XXXIII. 



INDEPENDENT MUSCULAR EXCITABILITY — AC- 

 TION OF CURARE— ROSENTHAL'S MODIFICA- 

 TION— POHL'S COMMUTATOR. 



1. Independent Muscular Excitability and the Action of 

 Ciu'are. — Curare paralyses the intramuscular terminations of the 

 motor nerves. — Apparatus. — Daniell's cell, induction machine, 

 two keys, five wires, shielded electrodes, scissors, fine-pointed 

 forceps, fine aneurism-needle, or fine sewing-needle fixed in a 

 handle, with the eye free to serve as an aneurism-needle, fine 

 threads, pithing-needle, i per cent, watery solution of curare, 

 hypodermic syringe or glass pipette. 



(a.) Arrange the battery and induction machine for an inter- 

 rupted current with a key in the primary circuit, and ' a Du Bois 

 key to short-circuit the secondary, as in Lesson XXXI. 2. 



(/>.) Destroy the brain of a frog, and by means of a hypodermic 

 syringe or a fine glass pipette inject into the ventral or dorsal 

 lymph-sac two drops of a i p.c. watery solution of curare. [The 

 curare of commerce is only partly soluble in water, but its active 

 constituent curarin is. Rub up i gram curare in loo cc. water 

 and filter]. The poison is rapidly absorbed. At first the frog 

 draws up its legs, in a few minutes it ceases to do so, and will he 

 in any position in which it is put, while the legs are not drawn up 

 on being pinched, and the animal lies flaccid and paralysed. 



(c.) Expose the heart, and observe that it is still beating. 



{(I.) Expose one sciatic nerve. 



(i.) Stimulate the sciatic nerve with interrupted shocks (faradisa- 

 tion) ; there is no contraction. 



(ii.) Apply the electrodes to the muscles ; they contract. 



Therefore curare ha^ paralysed some part of the motor nerves, hit 

 not the muscles. 



In curare poisoning the nerve-trunk itself is not inexcitable, but 

 the nerve-endings in the skeletal muscles are so afi'ected, i.e., 

 paralysed, as to prevent the excitatory state of tlie nerve being 

 propagated from the nerve to the muscle. The following experi- 

 ment proves this : — 



2. On what Part of the Motor Nerve does Curare Act ? 

 {a.) Induction apparatus as in the previous experiment. 



(h.) Destroy the brain of a frog. Expose the sciatic nerve and 

 the accompanying artery and vein on one side, e.g., the Z^/V, taking 

 great care not to injure the blood-vessels. Isolate the sciatic 



