Under all ordinary conditions the sequence of phenomena of asphyxia is the same—a stage of exaggerated breathing (hyperpnœa), a stage marked by the co-operation of muscles which are not called into action in tranquil breathing (dyspnœa), followed by the condition of asphyxia properly so termed. An animal whose supply of fresh air is cut off passes through these three stages, whether it be enclosed in a small space or in a very large one. It must, however, be noted that in asphyxia several factors combine in varying degrees. Carbonic acid is in excess in the blood, oxygen deficient. The nervous mechanism which regulates respiratory movements is thrown out of gear. Motor and inhibitory impulses are in conflict. It is important, if these complex phenomena are to be analysed, that one factor only should be altered at any given time. For example, carbonic acid may be allowed to increase in the air while a constant oxygen tension is maintained. Under these circumstances the dyspnœic contractions are much less marked. No convulsions follow. The paralysing action of carbonic acid predominates. Anæsthesia passes into complete unconsciousness. Death is tranquil. And this, speaking generally, is what happens in disease of the lungs. Asphyxia comes on slowly. The supply of oxygen is undiminished, but carbonic acid accumulates in the blood, acting as a narcotic poison which lowers the excitability of the nervous system, suspends consciousness, and slowly brings the vital activities to a standstill.