Tonus

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Abstract
Although it is now considered a truism to say that medicine must be based upon a sound knowledge of physiology, much confusion is still caused by the misuse of physiological terms through failure to understand the fundamental properties of living matter which these terms connote. In radiological literature no term is so much abused as the term “tonus,” especially in its application to the stomach. “Tonic action,” says Barclay,1 “is difficult to explain and there is no satisfactory definition of it.” Difficult to explain it certainly is; so are all fundamental things, the heart-beat for example. But it is not true to say that it cannot be satisfactorily defined. Bayliss2 defined it simply and succinctly when he said that it is the property of involuntary smooth muscle of remaining in a state of moderate contraction independent of any influence from the nervous centres, though capable of augmentation or diminution by the central nervous system. Cleland's definition of tonus as the ligamentous action of muscle is misleading, since peristalsis can be superimposed on tonus, whereas there is no peristalsis in a ligament. The most illuminating investigations into the nature of tonus have been made on the powerful adductor muscle of the bivalve molluscs such as the oyster.
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