Asthma does not appear to be influenced by the seasons, some authors claiming that it is most frequent in summer, while others maintain that the greatest number of cases occur in winter. Exciting Causes.--Bronchial asthma being a neurosis of the pneumogastric nerve, its exciting causes may be divided into those which act upon the nerve directly, and those which are reflected from more remote parts or organs. In the first class the irritant may act upon the nerve at its origin in the medulla oblongata or upon some part of its continuity. Various poisons, organic or inorganic, when introduced into the system may so change the character and composition of the blood as to interfere with the nutrition of the respiratory centre, and thus cause more or less embarrassment of respiration; but the attacks of dyspnoea due to these causes are more continuous than those of ordinary asthma, and are wanting in many of the symptoms which we have described as characteristic of that disease. These forms of dyspnoea are usually the result either of some constitutional disease or of some poison introduced into the system, both of which act by diminishing the proportion of red corpuscles in the blood. Of this we have examples in the dyspnoea sometimes observed in syphilis and malarial fever and in lead and mercurial poisoning--the so-called a. saturninum and a. mercuriale. It is true that there have been instances in which the paroxysms of asthma have come on at regular intervals and have yielded to quinine, but it is not regarded as proved that such cases were due to malarial poisoning (Sée). Enlarged bronchial glands pressing upon the pneumogastric nerve may cause asthma, and this explains why it is so frequent in children after attacks of measles and whooping cough (Williams and Biermer). Others have remarked that asthma is often coincident with hypertrophied tonsils (Schaeffer). In the great majority of cases the exciting cause does not act directly upon {192} the pneumogastric nerve, but upon the skin or some other remote organ, whence it is transmitted to the nervous centre and reflected back through the nerves of respiration to the bronchi. Biermer believes that the irritant in many cases, instead of being directly transmitted to the medulla oblongata, causes a fluxion to the exposed mucous membrane. He thinks that the absence of catarrhal symptoms is more apparent than real, the evidences of congestion being unappreciable during the early stages of the disease. According to Riegel,[12] the action of the irritant may be explained in one of three different ways--viz. 1st, both the spasm and the fluxion may be the common result of the irritant; 2d, the catarrh may cause the spasm; or, 3d, the spasm may secondarily produce catarrh. [Footnote 12: _Op. cit._, p. 256.]