For the relief of asthma an almost countless number of drugs have been suggested and tried. It is not my purpose even to enumerate these, but to call attention to those that are to day most frequently employed with success and to those with which I have had personal experience. Treatment of asthma is prophylactic and also is addressed to the immediate relief of the dyspnœa. In order to understand the mode of action of drugs in asthma, it is necessary to keep in mind its causes and the mechanisms within the body that are called into play in producing the phenomena of the disease. It is true that the bronchial tubes are congested during the dyspnœic attacks, but it is not probable that this is the cause of the difficulty of breathing. A true spasm of the bronchioles constitutes the essential change. This spasm is produced by contraction of the muscular tissue in the bronchial wall when excited by the nerves supplied to it. The initial irritation of the nervous system arises in only a moderate proportion of all cases within the bronchial tubes. In some cases of bronchitis, and in some cases of uncompensated heart disease with passive engorgement of the bronchial vessels the afferent nerves of the bronchi are irritated, and thus reflexly the efferent nerves and finally the bronchial muscles are excited. Immediate irritation of the efferent nerves may be the cause of spasm of the bronchi in these cases, although the action of drugs points to a reflex cause. More frequently the initial irritation is in some distant organ, whence reflexly the muscular nerves of the bronchi are excited. A common illustration of this mode of origin of bronchial spasm is seen in the cases of hay and rose fever that are complicated by asthma. The initial irritation is within the nose. In rare cases we find the efferent nerves to the bronchi excited by mental changes or changes originating within the central nervous system. In mild uræmic poisoning asthma occasionally occurs. Whether, in these cases, the uræmic poison primarily irritates the central nervous system and thus provokes the attack, or the peripheral nerves and bronchial muscles, is not known, but the former explanation seems probable. The mechanism, as you will notice, essential to the production of most asthmatic attacks, consists of the afferent nerves and primary irritation of their termini, of nerve centres, probably in the medulla, by which the irritation of the afferent nerves is reflected to the efferent nerves and, lastly, the efferent nerves and their endings in the muscles of the bronchi. For convenience we may name the first of these factors, the afferent nerve endings, the source of irritation; the second, the nerve centre, the reflector; and the third, the efferent nerves and bronchial muscle fibres, the focus of irritation.