PRACTICAL EXERCISES 811 



traction obtained by stimulating a neive with an induction machinfe 

 must not be confused with the break or make contraction caused by the 

 voltaic current. In the case of the induction machine, the break or 

 make applies merely to what is done in the primary circuit, not to what 

 happens to the current actually passing through the nerve. The 

 current induced in the secondary at make of the primary circuit is, of 

 course, both made and broken in the nerve made when it begins to 

 flow, broken when the flow is 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. 



(6) Repeat (a) with the muscle directly connected to the cell by thin 

 copper wires, or, better, unpolarizable electrodes (p. 731). 



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

 place a small piece of cork moistened with physiological salt solution 

 ("75 P er cent.) 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 a stop-watch the time the piece of cork 

 takes to travel over 10 millimetres. Then pour salt solution 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 so. It solution has been so hot as to injure 

 the cilia. 



4. 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 nerves right up to 

 the vertebral column. Lay their upper ends on electrodes and stimu- 

 late; the muscle of the ligatured limb will contract. This proves that 

 the nerve-trunks are not paralyzed by curara, since the poison has been 

 circulating in them above the ligature. The muscle of the leg which 

 was not ligatured will contract if it be stimulated directly, although 

 stimulation of its nerve has no effect. The ordinary contractile sub- 

 stance of the muscular fibres, accordingly, is not paralyzed. The seat 

 of paralysis must therefore be some structure or substance physiologic- 

 ally intermediate between the nerve-trunk and the general contractile 

 substance of the muscular fibres (p. 738). 



5. Graphic Record of a Single Muscular Contraction or Twitch. Pith 

 a frog (brain and cord), make a muscle-nerve preparation, and arrange 

 it on the myograph plate, as in i (b). Lay the nerve on electrodes 

 connected with the secondary coil of an induction machine arranged 

 for single shocks. Introduce a short-circuiting key (Fig. 240, p. 732) 

 between the electrodes and the secondary coil, and a spring key in the 

 primary circuit. Close the short-circuiting key, and then press down 

 the spring key with the finger. Let the drum off (fast speed) ; the 

 writing-point will trace a horizontal abscissa line. Open the short- 

 circuiting key, and then remove the finger from the spring key. The 

 nerve receives an opening shock, and the muscle traces a curve. Now 

 adjust the writing-point of an electrical tuning-fork (Fig. 287), vibrating, 

 say, 100 times a second, to the drum, and take a time-tracing below the 

 muscle-curve. Stop the drum, or take off the writing-point, the 

 mcment the time-tracing has completed one circumference of the drum, 



