The irritability of the air passages varies greatly in different individuals. Concentrated vapor may cause reflex spasm of the larynx and, consequently, obstructed breathing. This is the condition that leads to what is ordinarily called _respiratory collapse_. It is due to crowding—undue concentration—rather than excessive quantity of the anesthetic. If there were no superior laryngeal and trifacial nerves to warn the inexperienced or inattentive by closing the larynx to more of the anesthetic, real _respiratory paralysis_, which is apt to be fatal, and is due to direct toxic action of the anesthetic on the respiratory centre, might be more common. “Have I crowded the anesthetic?” is the first question that should be considered when there are signs of obstructed breathing. The jaw is rigid, the patient is almost awake, and yet the mask is lifted to admit more air. Paradoxical as it may seem, the jaw begins to relax, the breathing becomes free and the anesthesia at once more profound. The reason is simple. As long as the spasm of the larynx persists the anesthetic cannot readily pass the barrier to exert its physiological action. As soon as the spasm is overcome by admitting air the anesthetic can be freely inhaled. By observing the precaution to dilute the anesthetic generously with air pharyngeal irritation and laryngeal spasm can be avoided and an undisturbed narcosis secured. [Sidenote: Valve-action of the Lips] Sometimes, however, the obstruction is purely mechanical. It may be due to compression of the trachea by a shoulder brace. In aged individuals, after removing the tooth plate, progressively increasing cyanosis may be due to _valve-action of the lips_. Expiration is unhindered, but inspiration becomes impossible on account of collapse of the lips and cheeks. The difficulty is overcome by turning the head to one side and placing a spindle of gauze in the dependent angle of the mouth to keep the lips apart. [Sidenote: Recession of the Tongue] There are other cases in which the base of the tongue drops back into the oropharynx, and hinders breathing. There is a peculiar, noisy, “fluttering” respiration which indicates this condition. The jaw-grip, that is, pushing the jaw forward, is often insufficient. Most of us have been taught to use the wedge, mouth-gag and tongue forceps at once in such an emergency, but it is certainly desirable to escape this maneuver whenever possible. A naso-pharyngeal catheter, or breathing tube of soft rubber, passed through the nostril into the pharynx sometimes instantly relieves the obstruction. THE USE OF THE BREATHING TUBE. [Sidenote: Breathing Tube]