The most reasonable explanation offered of such an accident is that given by Leonard Hill. The attempts to inspire through an air way mechanically blocked cause an immense strain upon the heart muscle. The flow of blood in the lung capillary is hindered, and the right side of the heart becomes over distended with blood. Its musculature is further damaged by the fact that the blood in the coronary vessels is deficient in oxygen, and that a considerable dose of anæsthetic has already been absorbed. There is the further fact, not mentioned by Hill, that during the whole period of asphyxia the peripheral resistance is rising from vaso-constriction. Under circumstances such as these, it is obvious that _any_ heart must ultimately succumb, _no matter what anæsthetic is in use_. It is also obvious that with chloroform and ethyl chloride, which are themselves heart poisons, secondary syncope will happen much more readily than with ether or nitrous oxide, which are not; and that a heart with diseased musculature will fail quicker than a healthy organ. Secondary syncope is almost certainly the commonest fatal accident of anæsthesia. The reason why this fact is not more widely recognised arises from the natural instinct of any one who has suffered the misery and ignominy of causing a death under an anæsthetic, to attribute it to some cause beyond human control. The _essential_ cause of secondary syncope is failure to maintain a free air way, which cannot be styled unavoidable. Two consolations may, however, honestly be offered to the person who has acted as anæsthetist in a case of secondary syncope. Firstly, it is, in certain types of cases, very difficult indeed to maintain a free air way; and secondly, a heart with muscle degenerated from fatty or other changes, may give out after very little respiratory embarrassment. SYNCOPE FROM OVERDOSE. This is a more gradual affair than the two foregoing; and has been sufficiently dealt with in the chapter devoted to the Stages of Anæsthesia (see p. 36). C. REFLEX SYNCOPE.