Omnopon is composed of a mixture of several of the alkaloids derived from opium; the makers claim that it produces less after malaise than morphia alone. It may be given before anæsthesia in doses of ⅙–⅓ gr., either alone or combine with a small dose of scopolamine. It gives quite good results if not pushed to excess. HEROIN HYDROCHLORIDE This comparatively modern sedative is used by some surgeons in preference to morphia. A dose of ¹⁄₁₂ gr. is quite sufficient, three quarters of an hour before operation. Atropine should always be combined with it. * * * * * To young children, morphia should not be given, but atropine may be given freely. A child of twelve months tolerates a dose of ¹⁄₂₀₀ gr. quite well: one of six years, will take ¹⁄₁₅₀ gr. In ages ranging from 12 years upwards, greatly reduced doses of morphia may be given. No child under 15 years requires more than ¹⁄₁₂ gr. of morphia at most. CHAPTER VII. NITROUS OXIDE. Special Physiology. Upon the nervous system, nitrous oxide acts like other anæsthetics, but the stages of anæsthesia are passed through so rapidly that a second stage can hardly be distinguished. It is rare for struggling or excitement to be manifest, unless air or oxygen be admitted at the same time, when the effect which led Humphrey Davy more than a century ago to apply to nitrous oxide the name of “laughing gas” is very evident indeed. Upon the other systems of the body, nitrous oxide has little if any effect in itself. The essential point to remember in connection with nitrous oxide administered unmixed with air or oxygen is that there is an _inevitable element of asphyxia_. The larger part of the oxygen normally carried by the red blood corpuscles is eliminated and replaced by N_{2}O: oxygen starvation is therefore of necessity present. In other words, the “vicious circle of asphyxia” (_see_ Fig. 5) is entered, and muscular spasm is bound ultimately to appear. Moreover, the blood pressure rises very materially as a result of the lack of oxygen. That no harm results to the normal healthy patient from this rise is due to the fact that the gas does not in itself poison the heart muscle, which can therefore stand up to the extra strain of working against higher resistance, so long as the process is not carried to extremes. A heart muscle weakened by the action of chloroform would give out at once if exposed to such a test. Apparatus.