PRACTICAL EXERCISES 617 



the nerve made when it begins to flow, broken when the flow if 

 over ; the shock induced at break of the primary is also made and 

 broken in the nerve. And although make and break of the actual 

 stimulating current come very close together, the real make, here, too, 

 is a stronger stimulus than the real break. 



(ti) Repeat (a) with the muscle directly connected by thin copper 

 wires, or, better, unpolarizable electrodes (p. 542), to the cell. 



3. Mechanical Stimulation. Pith a frog. Cut away the anterior 

 portion of the body, dissect out one sciatic nerve, and separate the 

 leg to which it belongs from the other. Pinch the end of the nerve 

 or prick the muscles, and they contract. 



4. Thermal Stimulation. Touch the nerve of the same prepara- 

 tion with a hot wire ; the muscle contracts. The nerve is killed at the 

 point of contact, but can be again stimulated by touching it with the 

 wire lower down. 



5. Chemical Stimulation. (a) Cut off the injured portion of the 

 nerve used in 3 and 4. Apply to the cut end a crystal of common 

 salt, or let the nerve dip into a watch-glass containing a saturated 

 solution of salt. In a short time the muscles supplied by the nerve 

 begin to twitch, and soon enter into irregular tetanus. Take a 

 tracing of the contractions. Cut off the piece of nerve in contact 

 with the salt, and the tetanus stops. This shows that the seat of 

 irritation is the portion of the nerve into which the salt has pene- 

 trated, and from which water has been withdrawn by osmosis. Con- 

 traction can also be caused by applying the salt directly to the 

 muscles. 



(b) Wrap the leg in blotting-paper moistened with normal saline, 

 and expose the nerve to the vapour of strong ammonia ; it will be 

 killed, but not stimulated, for the muscles will not contract. Expose 

 the muscles themselves to the ammonia, and contraction will occur. 

 Accordingly muscle is stimulated by ammonia, while nerve is not. 



6. Ciliary Motion. Cut away the lower jaw of the same frog, and 

 place a small piece of cork moistened with normal saline on the 

 ciliated surface of the mucous membrane covering the roof of the 

 mouth. It will be moved by the cilia down towards the gullet. Lay 

 a small rule, divided into millimetres, over the mucous membrane, 

 and measure with the stop-watch the time the piece of cork takes to 

 travel over 10 millimetres. Then pour normal saline heated to 

 30 C. on the ciliary surface, rapidly swab with blotting-paper, and 

 repeat the observation. The piece of cork will now be moved more 

 quickly than before, unless the normal saline has been so hot as to 

 injure the cilia. 



7. Direct Excitability of Muscle Action of Curara. Pith the 

 brain of a frog, and prevent bleeding by inserting a piece of match. 

 Expose the sciatic nerve in the thigh on one side. Carefully separate 

 it, for a length of half an inch, from the tissues in which it lies. Pass 

 a strong thread under the nerve, and tie it tightly round the limb, 

 excluding the nerve. Now inject into the dorsal or ventral lymph- 

 sac a few drops of a i per cent, curara solution. As soon as paralysis 

 is complete, make two muscle-nerve preparations, isolating the sciatic 



