The preventive treatment consists chiefly in following the other rules set forth above for the prevention of asphyxia with such faithful care, that the patient never enters into the vicious circle of asphyxia of which stridor is so prominent a feature. Patience in the induction stage--the avoidance of _forcing_ the anæsthetic upon the patient--is a safeguard not to be forgotten. Once the condition has arisen, it saves time to _withdraw the anæsthetic_ altogether, and to allow the patient to breathe nothing but fresh air. Brisk friction of the lips with a rough towel often does good, presumably by setting up a “cross reflex.” In severe cases, a most valuable measure is the inhalation of pure oxygen, a cylinder of which should always be at hand in the operating theatre. Even an obstructed air-way will convey enough undiluted oxygen to reduce the venosity of the blood, and so cut across the “vicious circle.” So much for the treatment of the early stages of asphyxia, the more advanced stages constitute one of the “accidents of anæsthesia,” and are dealt with in Chapter XVI. CHAPTER IV. METHODS OF ANÆSTHETISING. Certain terms such as “open method,” “closed method,” etc., are used in describing different systems of anæsthetising, and it will save time later if these are now defined. The OPEN METHOD is one in which the drug is dropped or poured upon a fabric stretched on a mask which does not lie in close apposition to the face. If the student will experiment with such a mask as Schimmelbusch’s, he will find that by no effort can he make its whole circumference touch his face at the same time. Anæsthetics vapourised from such masks must of necessity be inhaled freely diluted with fresh air. These masks are only suitable for use with chloroform. The PERHALATION METHOD.--This term is not used often, but it is the most strictly correct name to give to the process commonly called “open ether.” If the student will examine Bellamy Gardner’s open ether mask (Fig. 27) he will find that it is deliberately shaped to lie over its entire circumference in close apposition to the face of the average patient. In actual use it is well however to make sure of this apposition by the use of a ring of gauze as shown in Fig. 28A. Upon the mask is stretched gauze of a thickness just as great as will permit free respiration to take place through its layers. The whole bulk of the respired air must pass _through the fabric_, none escaping between the face and mask. The term “SEMI-OPEN” is applied to various methods now rarely seen. One of the best known of these was the anæsthetic cone, still used by a few for C.E. mixture (_see_ Fig. 45).