During postmortem rigidity a rapid disappearance of motor end plates occurs in human skeletal muscle, associated with a considerable local increase in hydrogen ion concentration. This increase is due mainly to a local accumulation of lactic acid, which usually reaches a concentration of from 0.05 to 0.3 per cent1 in rigor mortis. The possible causal relationship between locally increased acidity and end plate degeneration was tested by Carey2 of the Department of Anatomy, Marquette University, who studied the morphologic effects of intramuscular injections of lactic acid in white rats.
Under pentobarbital sodium anesthesia the right sternomastoid muscle was removed as a control and run through the gold technic3 and other histologic methods. Following this removal 0.1 cc. of 0.05 to 0.3 per cent lactic acid in isotonic solution of sodium chloride was injected into the opposite sternomastoid muscle. The injected muscle was removed for microscopic study from