'behind' and τόνος, tonos, 'tension') is a state of severe hyperextension and spasticity in which an individual's head, neck and spinal column enter into a complete "bridging" or "arching" position.

It has been shown to occur naturally in birds, snakes suffering from advanced boid inclusion body disease, and placental mammals, among existing animals; it is observed in some articulated dinosaur fossils.

[citation needed] Severe "arching" (hyperextension) occurs due to stimulus by the anterior reticulospinal tract caused by the loss of the balancing inhibitory counter-stimulus of corticoreticular fibers, which normally act upon the pons reticular formation.

[citation needed] Opisthotonus is also described as a potential CNS symptom of heat stroke along with bizarre behavior, hallucinations, decerebrate rigidity, oculogyric crisis, and cerebellar dysfunction.

[4] Sir Rudolph Peters, in Oxford, introduced thiamine-deprived pigeons as a model for understanding how thiamine deficiency can lead to the pathological-physiological symptoms of beriberi.