Natural (reflex) contractions of the skeletal muscles of vertebrates are apparently of two different types: "phasic" and "tonic."
The properties of the contractions of the phasic type, the voluntary contraction of man, for example, are clearly those of a neuromuscular tetanus.
The contractions of the tonic type, the so-called decerebrate rigidity of mammals, for instance, are characterized, especially, by the smallness of the phonomyographic and the electromyographic oscillations of the active muscles, their apparent indefatigability and by their plasticity which enables them to maintain the same contractile tension in widely different lengths.
The neuromuscular determinism of muscle tone is still a matter of controversy among physiologists. Two theories of its mechanism have been proposed and are still defended. For one group of investigators (Bottazi, de Boer, Langelaan, E. Frank, J. I. Hunter and Ranson),1 the differences between the tonic and phasic types of sustained contraction result from a radical
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