TON US 541 



when the stimuli are separated by intervals, the latter are clonic and consist of a 

 short response to each stimulus. 



Since the leg is maintained in a different position, even when it is loaded, it is 

 clear that a particular length of muscle may be in equilibrium with different loads ; 

 in other words, the same length of fibre may have different tensions, just as we 

 saw in the case of the involuntary muscle. 



Another point of interest is that the reflex contractions of the de-afferented 

 muscle are fatigued sooner than those of the muscle in its normal state. 



The mechanism must consist in each particular length of the muscle being able, 

 in some way, to stimulate the receptors of this muscle in such a manner as to 

 maintain the degree of contraction at this level. 



We must next consider certain experimental facts which show that this reflex 

 tonic contraction is a different thing from a steady contraction of the same height, 

 produced by the application of repeated induction shocks to the cut' nerve at a 

 sufficient rate to give a fused curve, or from the ordinary spinal reflex described in 

 Chapter XVI. 



Inhibition of T onus. It will be remembered that Sherrington showed that, in 

 the case of spinal reflexes, by appropriate relative strength of stimuli applied to 

 different nerves, one excitatory, the other inhibitory, any degree of reflex 

 contraction of the vasto-crureus can be obtained. Curves illustrating this fact 

 are given in Figs. 118, 122, and 123, on pages 410-415 above, and a further one in 

 the upper curve of Fig. 174. But, supposing that, instead of the ordinary reflex 

 contraction, which can be reduced to any desired extent by different strengths of 

 the stimulus to the inhibitory nerve, we take the tonic contraction of decerebrate 

 rigidity and attempt to reduce it to different degrees by varying the strength of 

 the stimulus of the same inhibitory nerve. It was shown by Sherrington (1909, 2, 

 pp. 256 and 257) that no algebraical summation is possible ; any strength of 

 stimulus which has any action at all produces a gradual fall in the height of the 

 tonus, which fall continues until complete relaxation results, if the stimulation is 

 continued long enough. In other words, instead of falling rapidly to a certain 

 point and remaining there, the tonus completely disappears. The only difference 

 between the effect of strong and weak stimuli is the rate of fall, as is seen in the 

 lower curves of Fig. 174. One is again reminded of the removal of the "catch" 

 in the inhibition of the adductor muscle of the scallop, although the mechanism is 

 peripheral there, central here. 



Frohlich and Meyer (1912) again, have noticed phenomena with tetanus toxin 

 which lead them to regard the relaxation of mammalian muscle as being, in some 

 way, directly under the control of the central nervous system. If a particular 

 segment of the spinal cord is poisoned with this toxin, the muscles supplied by this 

 segment enter gradually into a state of shortening. In this state, the metabolism 

 of the muscle appears to be abnormally small ; glycogen accumulates in it. It gives 

 no muscle sound and no vibration on the string galvanometer. If, however, the 

 muscle is passively stretched by pulling upon it, the muscle sound is heard and the 

 galvanometer shows the characteristic vibratory current of action of voluntary 

 tetanus. On the view suggested above, it might be supposed that the process 

 (production of lactic acid) which is responsible for the increase of tension has 

 become, so to speak, permanent ; hence the state of surface tension does not 

 disappear spontaneously, owing to the removal of the acid under the influence of 

 oxygen, but requires some nervous influence to set the necessary mechanism into 

 play. The phenomenon differs, however, from those of involuntary muscle in that, 

 in the cases under discussion here, it is of central origin, as remarked in the 

 preceding paragraph. 



Metabolism. In the experiments of Frohlich and Meyer, as given above, it 

 was noted that the metabolism was unusually low. Now Roaf (1912) has 

 described experiments in which he found that the carbon dioxide output and also 

 the oxygen intake were no greater in the state of decerebrate rigidity than in a 

 subsequent period in which the contraction was abolished by the use of curare. 



It should be stated that Lovatt Evans found the metabolism to be less under curare than 

 in decerebrate rigidity. Of course, care was taken in both sets of experiments to prevent fall 



