_Firstly, cases in which an asphyxial element already exists will have their condition greatly aggravated by the substitution of N_{2}O for the oxygen in their blood._--Already caught in the vicious circle of asphyxia, nitrous oxide would but push them deeper into the vortex. Examples of such cases are patients suffering from tumours or inflammatory swellings in the neck which are pressing upon the air passages. In passing, one may note that it might be the desire of the surgeon to submit an individual case falling into this group to the operation of tracheotomy for the immediate relief of the condition. The short space of time required for this little operation might well tempt the unwary to choose nitrous oxide as the anæsthetic, and in point of fact such an error of judgment has more than once been made with fatal results. _Secondly, no patient suffering from any condition which will be aggravated by a sudden rise of blood pressure, should be submitted to nitrous oxide undiluted by oxygen._--Examples of such conditions are cases of _dilated right heart_ with weakened cardiac musculature. Such hearts could not be expected to work against a peripheral resistance suddenly raised, say, from 120 mm. of Hg, to 180 or even 200 mm.--figures well within the possible in deep gas anæsthesia. Similarly, so great an increase of pressure would be dangerous to a patient suffering from an _aneurysm_, or from _extensive arterio-scelorosis_ with high blood pressure. It will be noted that the above warnings are limited to the use of pure nitrous oxide, that is, N_{2}O unmixed with oxygen. The extent to which the dangers referred to can be met by the admixture of oxygen in the manner to be described in the next chapter is largely a matter of the skill and experience of the administrator. Nasal Methods. The object of using this route is to be able to continue the administration throughout the period in which the dentist is doing his work. The essentials of a suitable apparatus are:-- 1. A malleable nosepiece which can be closely adapted to the nose. 2. Two supply pipes from bag to nosepiece. 3. Some means of admitting air to the stream of gas. 4. A two-gallon bag. 5. A supply of gas. 6. A mouth cover with an expiratory valve only. The patient is instructed to breathe in through the nose, but to expire through the mouth. The gas is supplied under some pressure and the mouth cover ensures that no air is inspired by that route. Some patients find it easier to conduct both inspiration and expiration through the nose, and for their benefit an expiratory valve is also provided in the nosepiece. After unconsciousness has supervened, nearly all patients begin to-and-fro nasal breathing. The mouth cover may then be removed, and if it be desired to economise gas, the expiratory valve in the nosepiece may be thrown out of action. [Illustration: FIG. 17.--Ash’s Modification of Paterson’s Nasal Gas