The dyspnoea of bronchitis comes on more gradually, the attacks being dependent upon a variety of accidental circumstances; whereas the asthmatic paroxysm usually occurs quite suddenly in the night without any apparent cause. The cough in bronchitis is severer and the expectoration more abundant than in asthma; the latter is also different in quality, becoming purulent as the disease advances, whereas in asthma it seldom loses its mucous character. These points of difference and the presence of the other symptoms of bronchitis are sufficient to differentiate that disease. 2. Emphysema is frequently associated with asthma, either as a cause, as is believed by many, or as an effect of that disease. It is often exceedingly difficult to determine whether the emphysema when present is the cause of the dyspnoea (symptomatic asthma), or whether the inflation of the air-cells and other symptoms are not the result of the bronchial spasm: a careful inquiry into the history of the case will often decide the question. The points of difference between the two diseases are very similar to those to which we have just called attention as the distinguishing features between the dyspnoea of bronchitis and the true asthmatic paroxysm. The suddenness with which the attack comes and goes, the severity of the symptoms compared with the insignificance of the local lesions, the absence of dyspnoea in the intervals between the attacks (in uncomplicated cases), are all the reverse of what is observed in emphysema. In that disease the attacks develop more gradually; there is always more or less shortness of breath, and the evidences of changes in the structure of the lung are quite marked. 3. Dyspnoea resulting from cardiac disease is often very severe, but may be distinguished from bronchial asthma by the presence of the various murmurs and other physical signs by means of which that class of diseases is recognized. The asthmatic paroxysm, as a rule, comes on when the patient is most quiet, usually during sleep. The attack of cardiac dyspnoea, on the contrary, is always brought on or aggravated by physical exertion, mental excitement, or some other apparent cause. In asthma the respiration during {199} the intervals between the paroxysms is quite natural; in cardiac dyspnoea there is always more or less embarrassment. Pain in the region of the heart, in many cases quite severe and extending down the left arm, may direct attention to that organ as the source of the dyspnoea.