If one were asked the question "What is the function of cough?" the reply would probably be that it is to clear the respiratory tract, of either an irritant or a block to the free passage of air in and out of the tracheobronchial tree. Thus, the general conception would be that the act of coughing results in an expulsive force which tends to drive whatever substance is producing the irritation, or block, up and out of the respiratory tract. The purpose of the experiments under consideration was to determine, whether or not this was the sole result of the act of coughing on material in the tracheobronchial tree. For, it had occurred to one of us,1 as doubtless to many others, that cough, under certain circumstances, might actually be the means of spreading the material which produced the irritation deeper into the finer ramifications of the pulmonary tree