GROUP XVIII  . Nitrites.—This group contains amyl nitrite, ethyl nitrite, methyl nitrite, nitroglycerin, sodium and potassium nitrites, erythrol-tetranitrate, and many other compounds containing nitrous or nitric acid . The latter becomes reduced to nitrous in the body, and thereby exercises its characteristic effects . These consist chiefly in an action upon non-striped muscle, vasomotor centres, blood vessels and the blood . When they are given by inhalation or by the mouth their first effect is to produce marked dilatation of the small arteries, with a fall of blood-pressure and a greatly increased rapidity of the heart's action . At the same time the non-striped muscles slightly lose their tonicity, and when very large doses are given the haemoglobin of the blood becomes converted into the chocolate-coloured methaemoglobin . The volatile members of the group act much more rapidly and more transiently than the others .