Physiology: H.hhoagland Quantitative Aspects of Tonic Immobility in Vertebrates

I.-"Death feigning," otherwise "playing 'possum") or even "animal hypnosis," better termed tonic immobility,l is shown to a striking degree by a great variety of animals.2'3'4'5 Certain forms from planarians to mammals may be caused to assume a state of rigid immobility by tactual stimulation of particular regions of the body or by other forms of manipulation. In the case of many of the higher vertebrates, including some mammals, the phenomenon has been said to be produced as a form of escape mechanism. Some writers have considered the "feint" to possess survival value and have spoken of it as an instinct of death feigning.6 Immobility periods in the case of most animals are characterized by certain physiological criteria, principal of which are a hypertonicity of the skeletal musculature and a non-reactiveness to environmental disturbances. The periods vary from a few seconds to several hours, depending on factors about to be discussed. II.-In 1923 Crozier and Federighi7 studied the effect of temperature on the durations of periods of tonic immobility of the isopod Cylisticus convexus. They had previously noted that when the animal was reimmobilized as soon as it had spontaneously recovered, the durations of the successive periods tended first to increase and then to decrease again in cyclical fashion, and that typically the cycle continued to repeat itself rhythmically as long as the process of reimmobilization was continued. They found that when the durations of the cycles were plotted against