622 A MANUAL OF PHYSIOLOGY 



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 in the muscles. That it is not usually in the nerve- 

 endings nor in the nerves may be shown by inducing fatigue of the 

 finger for voluntary contraction in the same way, and then stimu- 

 lating the median nerve at the bend of the elbow by sponge elec- 

 trodes. The usual seat of fatigue for voluntary muscular contraction 

 must therefore be in the spinal cord or brain, and as we have no 

 reason to believe that the nerve-fibres of the central nervous system 

 are essentially different from peripheral nerve-fibres, we conclude that 

 the fatigue is in the nerve-cells or the network of fibrils around them 

 (p. 662). 



14. Influence of Veratria on Muscular Contraction. Arrange a 

 drum as in Fig. 192 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 veratria. In a few 

 minutes make two muscle-nerve preparations from the posterior limbs. 

 First put the preparation from the unligatured limb on the myograph 

 plate. Lay the nerve on electrodes connected through a short-circuit- 

 ing key with the secondary of an induction machine arranged as in 

 Fig. 192. Put the writing-point on the drum and set it off (fast 

 speed). Open the short-circuiting key till the nerve has been once 

 stimulated, then close it again. The curve obtained differs from a 

 normal curve, in that the period of descent (relaxation) is exceedingly 

 prolonged. Now connect the preparation from the ligatured limb 

 with the lever, and take a tracing of a single contraction. Put on a 

 time-tracing with the electrical tuning-fork (see Figs. 177, 178). 



15. Effect of Suprarenal Extract on Muscular Contraction. 

 Proceed as in 14, but instead of veratria inject a watery solution of 

 the suprarenal capsules (calf, sheep, dog, etc.). The curve of the 

 gastrocnemius acted upon by the extract is prolonged as in veratria 

 poisoning, although not to such a great extent. (For the action of 

 suprarenal extract on the smooth muscle of the bloodvessels, see 

 p. 189.) 



1 6. Measurement of the Latent Period of Muscular Contraction. 

 Use the spring myograph (Fig. 167, p. 560), raising it on blocks of 

 wood. Smoke the glass plate over a paraffin flame, or cover it with 

 paper, and smoke the paper. Connect the knock-over key of the 

 myograph with the primary circuit of an induction coil. Pith a frog, 

 and make a muscle-nerve preparation. Arrange it on the myograph 

 plate. Place electrodes below the nerve as near the muscle as 

 possible, and connect by a short-circuiting key with the secondary. 

 Bring the writing-point in contact with the smoked surface of the 

 spring myograph, so as to get the proper pressure. See that the 

 writing-point of the tuning-fork is in the right position for tracing 

 time. Then push up the plate so as to compress trie spring, till the rod 

 connected with the frame which carries the plate is held by the catch. 



