PRACTICAL EXERCISES 709 



secondary of an induction machine arranged for tetanus. With a 

 camel's hair brush moisten one of the nerves between the electrodes 

 and the muscle with a mixture of equal parts of ether and alcohol, 

 diluted with twice its volume of water, to abolish the conductivity. 

 Or put the mixture in a small bottle, in which dips a piece of filter- 

 paper. The projecting end of the filter-paper is pointed, and the 

 nerve is laid on the point. As soon as it is possible to stimulate 

 the nerves without obtaining contraction in this muscle, proceed to 

 tetanize both nerves till the contracting muscle is exhausted. If the 

 other muscle begins to twitch during the stimulation, more of the 

 ether mixture must be painted on the nerve. As soon as the stimula- 

 tion ceases to cause contraction in the non-etherized preparation, 

 wash off the mixture from the other nerve with physiological salt 

 solution, and soon contraction may be seen to take place in the 

 muscle of this preparation. This shows that the nerve-trunk is 

 still excitable. Now, both nerves have been equally stimulated, 

 and therefore the exhaustion in the non-etherized preparation was 

 not due to fatigue of the nerve-fibres, but of something between them 

 and the contractile substance of the muscle. 



10. Seat of Exhaustion in Fatigue for Voluntary Muscular Con- 

 traction. Support the arm, extensor surface downwards, on a rest 

 such as that shown in Fig. 262, or Fig. 240, p. 650, and connect 

 the middle finger of one hand, by means of a string passing over 

 a pulley on the edge of a table, with a weight of 3 or 4 kilos. The 

 string is attached to the finger by a leather collar surrounding the 

 second phalanx of the finger, but allowing free movements of the 

 joints. The extent of the vertical movements of the string (and 

 therefore the work done) may be registered on a drum by a writing- 

 point connected with it, the whole arrangement forming what is called 

 an ergograph. Two collar electrodes (strips of copper covered with 

 cotton -wool soaked in salt solution, and bent to a circular form) are 



E laced on the forearm, and connected through a short-circuiting 

 ey with the secondary coil of an induction machine arranged for 

 tetanus (p. 184), and having a battery of two or three good dry cells 

 or of four or five Daniell cells, coupled in series,* in its primary 

 circuit. The middle finger is now made to raise the weight re- 

 peatedly by vigorous contractions of the flexor muscles until at 

 length a failure occurs. At this moment the short-circuiting key is 

 opened, and the flexor muscles stimulated electrically. They again 

 contract, and raise the weight, therefore the seat of exhaustion in 

 voluntary muscular effort is not usually in the ordinary contractile 

 substance of the muscles. That it is not usually in the nerves 

 may be shown by inducing fatigue of the finger for voluntary 

 contraction in the same way, and then stimulating the median nerve 

 at the bend of the elbow by sponge electrodes. The usual seat of 

 fatigue for voluntary muscular contraction must therefore be in the 

 spinal cord or brain. 



11. Influence of Veratrine on Muscular Contraction. Arrange a 

 drum as in Fig. 261. Pith a frog (brain only), expose the sciatic 

 nerve in one thigh, and isolate it for inch from the surrounding 

 tissues. Pass under it a strong thread, and ligature everything 

 except the nerve. Now inject into the dorsal or ventral lymph-sac 

 a few drops of o - i per cent, solution of sulphate of veratrine. In a 

 few minutes make two muscle-nerve preparations from the posterior 

 limbs. First put the preparation from the unligatured limb on the 



* I.e, } the copper of one cell connected with the zinc of the next. 



