or lobar pneumonia may be seen at any period of childhood, but in infancy is comparatively rare. Up to the end of the second year inflam mation of the lung usually assumes the catarrhal form, and even in the third year pneumonia is more often catarrhal than croupous. After the third year both forms of the disease are about equally common, and with each succeeding year inflammation of the lung, if it occurs, is more and more likely to be of the croupous variety.
late a tendency has been growing to look upon croupous pneumonia as an acute general disease, of which the pulmonary consolidation is the anatomical expression, and no longer to regard it as a mere local inflammation. Some observers have compared it to acute rheu matism and tonsillitis. Others, who see in the affection the effects of a spe cial poison, have even placed it in the same class with typhoid fever and other similar specific distempers.
That the disease is a general one, with a marked local manifestation, seems to be evident, for the general symptoms are not proportioned in severity to the extent of lung surface involved ; they may precede by some days any evidence of local mischief, and the highest elevation of tempera ture is often reached before the point of most complete consolidation is arrived at. Moreover, the character of the symptoms differs in many re spects from the ordinary type of constitutional disturbance set up by a local injury : head symptoms are more common, sweating is more frequent, and a herpetic eruption is an ordinary phenomenon. Again, the morbid exudation, which is the chief local expression of the disease, is of a kind peculiar to pneumonia, and cannot be produced by ordinary inflammatory agency. 'p ency. Still, although the affection may be a general one, it does not follow, as some observers are disposed to believe, that it ought to be classed amongst the diseases which result from specific infection. There are no doubt some facts which seem to favour this view. Thus, pneumonia has been occasionally known to occur in epidemics, and in some outbreaks facts have been noted which seem to point to personal communication of the disease by contagion. The illness sometimes appears to be preceded
by a prodromal interval, and to pass through a stage of invasion before local symptoms are manifested; it runs a definite, uniform course ; is often accompanied by complications which assume different degrees of prom inence in different outbreaks, and its type varies in severity, the rate of mortality being higher in some epidemics than it is in others. In all these features the disease seems to incline to the class of acute specific maladies. The question whether or not the illness can be set up by impressions of cold, is one of great importance, for if it can arise from a simple chill, the disease can have no pretensions to be the consequence of a specific poison. There is a conflict of testimony upon this point. It is said that pneumo nia is most frequent in the tropics, and diminishes in prevalence as the distance from this zone increases. It is not especially common in cold latitudes ; and Koch in his cases failed to trace any relation between the attack and the external temperature. Other observers, however, have no ticed a connection between the illness and meteorological conditions ; and there is no doubt that in seasons where the temperature is changeable and the weather damp the disease is more common than at times when the temperature is uniformly high or uniformly low. Biach states, as a result of his observations, that the coincidence of rapid atmospheric depression, a low temperature, and sudden changes of temperature tends to produce the disease.
Perhaps in the present state of our knowledge it may be sufficient to class pneumonia with tonsillitis, and, indeed, it bears a great resemblance to that disease in the conditions under which it appears to originate. In addition to cold, bad drainage seems to have a powerful influence in excit ing the malady. Many mysterious cases of pneumonia arising in schools have been finally traced to contamination of the air of dormitories by sewer gas, and have ceased after measures have been taken to rectify the faulty condition of the drains.