DESCRIPTION:--A--Nose-Cap Attachment with Stopcock for Air and Gas, and with Inspiratory and Expiratory Valves and Shutter. B--Nose-Cap. C--Sliding Clip on India-rubber Tubings. D--Bifurcated Mount. E--Bag Mount. F--Gas Bag Compressor. G--Gas Bag. H--India-rubber Tubing. MC--Mouth Cover. This is fitted with an expiratory valve, and should be used at the same time as the Nose-Cap. By using the two together patients are more quickly anæsthetised than they would be if only the Nose-Cap were used, and the Nitrous Oxide is economised.] Air must be admitted in limited quantity through the tap after the first thirty seconds or so; by a judicious regulation of this mechanism, anæsthesia may be prolonged for five or ten minutes. Facility with nasal gas comes only after some considerable practice. Fig. 17 shows Ash’s No. 3 Patented Nasal Inhaler, which admits of Air or Gas being administered either by the Naso-Oral Method, or by to-and-fro nasal breathing. _Air_ only is admitted when the Shutter A is open and turned to the right as far as it will go. _Nitrous Oxide_ is admitted when the Shutter A is open and turned to the left as far as it will go, and the patient will breathe it in-and-out through the nose. _For the Nasal Oral Method_, close Shutter A and turn it to the left as far as it will go. This will cause the patient to inhale through the nose and exhale through the mouth. Nasal attachments are provided with most of the gas-oxygen apparatuses mentioned in the next chapter. With them good results can be obtained with much greater ease. For a more detailed account of nasal methods, the student is referred to works devoted entirely to Dental Anæsthesia. CHAPTER VIII. NITROUS OXIDE AND OXYGEN. The account given in the previous chapter of anæsthesia by nitrous oxide and air will have convinced the student that it is a somewhat inelegant method with a limited sphere of usefulness. The reason is obvious. In atmospheric air, oxygen exists only in the proportion of about one to four of nitrogen. To sustain life it is therefore necessary to admit to the anæsthetic mixture an amount of air which leaves too little room for the anæsthetic factor--nitrous oxide. If, however, pure oxygen be used, the nitrous oxide is diluted to a much less degree, and far better results are obtained. The exact scope for gas-oxygen anæsthesia cannot at present be defined with certainty. The work of Crile, and the experience of the war have done much to enlarge it. We may say that the following are definite indications for its use:-- (1) Minor operations lasting 5–15 minutes, particularly if performed on out-patients. (2) Operations of any variety upon the subjects of severe shock. (3) Operations upon patients suffering from acute sepsis. (4) Operations repeated upon the same subject at short intervals.