_The phenomena_ seen in nitrous oxide anæsthesia are so different from those of any other that a few words must be said about them. Within a few seconds of the inhalation beginning, the colour of the patient shows evidence of the presence of the gas in his blood. The normal complexion changes first to a dull pink, and very rapidly to the definite blue of cyanosis. The _eye symptoms_ are of the utmost value. Very early the pupil begins to dilate, and the eyeball tends during the first twenty or thirty seconds to rotate as if the patient were looking for some object in his field of vision. In full anæsthesia the eyeball, however, comes to rest, usually pointed downwards. The pupil is widely dilated, the conjunctival reflex is almost or even entirely abolished, but the corneal reflex is still brisk. _The respiration_ tends to become steadily deeper and more frequent, and in the later stages stertor at least, if not stridor usually develops. _The muscles_ under ordinary nitrous oxide anæsthesia are rarely entirely relaxed, but the limbs hang motionless, and it is only if an attempt be made to move them into some abnormal position, that one appreciates the persistence of muscular tone. A phenomenon peculiar to nitrous oxide anæsthesia is observed in its deepest stage. Designated as _jactitation_, it consists in a tremor beginning in the limbs, but spreading from them to the trunk if its development is allowed to proceed. It is a finer movement than that described under “ether tremor” (page 38), and wholly different in type from the athetosis referred to on page 38. Jactitation is almost wholly an asphyxial phenomenon, and is therefore definitely an indication that the process of oxygen starvation has been carried as far as is permissible. _The signs of fully developed nitrous oxide anæsthesia_ then are:-- 1. Deep regular snoring respirations. 2. Dilated pupils. 3. Rotation of the eyeball downwards. 4. Loss of conjunctival but persistence of corneal reflex. 5. A colour of the skin definitely blue, but not blackish blue. 6. The commencement of jactitations. _The signs of overdose_ are:-- 1. An enormously dilated pupil not re-acting to light. 2. Loss of corneal reflex. 3. A blackish blue colour. 4. Jactitations fully developed. 5. Failing respirations. The final arrest of respiration in nitrous oxide anæsthesia is usually painfully sudden. Upon the respiratory side the only warning is one or two gasps, and even that is sometimes absent. The paralysed pupil and the jactitations are the most useful signs of overdose.