112 PRACTICAL PHYSIOLOGY 



curve the first process preponderates over the second and the 

 height of the contraction decreases, but as soon as the two pro- 

 cesses exactly balance each other a uniform level is maintained 

 for hundreds of contractions. The probable seat of these pro- 

 cesses will be referred to after the next experiment has been 

 performed. 



In order to obtain a record of the contractions of the gastro- 

 cnemius with its circulation intact, arrange the apparatus for 

 stimulating the sciatic nerve with maximal induction shocks, using a 

 simple key in the primary circuit. The cerebrum of the frog must 

 be destroyed and the muscle-nerve preparation made without 

 causing bleeding. The cerebral hemispheres are destroyed by 

 compression, leaving the medulla and spinal cord intact, and the 

 gastrocnemius is prepared in the usual way. A string ligature is 

 placed beneath the gastrocnemius and tied tightly round the upper 

 part of the tibio- fibula and the remaining muscles ; the leg is then 

 cut through below the ligature. The whole frog is placed belly 

 downwards on the myograph-board, a strong pin is pushed through 

 the lower end of the femur and driven firmly into the cork. A 

 piece of moistened flannel is then pinned down over the trunk to 

 prevent the contractions of the muscles of the trunk from disturbing 

 the lever connected with the gastrocnemius. The skin over the middle 

 of the thigh is divided longitudinally for a short distance, the muscles 

 carefully separated and the sciatic nerve exposed and freed ; the nerve 

 is gently raised by slipping a thread beneath it, and the electrodes, 

 insulated from the underlying muscles by a small piece of cork, are 

 placed beneath the nerve. It is essential that the nerve should not 

 be injured and should be kept properly moistened throughout the 

 experiment. The muscle is suitably weighted and just after-loaded. 

 The nerve is stimulated by a maximal shock every five seconds, and 

 the contractions recorded on a drum revolving at the slowest possible 

 rate (Fig. 108). It will be seen that the height of the contractions, 

 although increasing at first, gradually falls off until at the end of 

 about 200 contractions it reaches a uniform level, which represents 

 about 85 per cent, of the original height and was then maintained 

 with scarcely any alteration for three-quarters of an hour. This 

 curve, therefore, is identical in general form with that obtained by the 

 ergograph. We see at first an increase, then a fall, and then a constant 

 level of contraction, representing probably the equilibrium between 

 two opposite processes, which must in this case be affecting some 

 part of the peripheral nerve and muscle. The actual seat of this 

 peripheral change is not absolutely certain (see further Experiments 

 in Chapter XVII). 



Now cut through the leg in the middle of the thigh, so as to destroy 

 the circulation through the gastrocnemius and continue the stimu- 

 lation (Fig. 109). It will be seen that the height of the contractions 

 rapidly and continuously decreases, and that at the end of about 320 

 contractions the muscle is no longer able to lift the lever off the 

 after-loading screw. 



