In discussing this aspect of the question before the Section I shall address myself only to the clinical exami¬ nations of the blood possible to the general practitioner. The bacteriologist has been able to isolate the pneumococ¬ cus from the blood of pneumonic cases, especially such as are complicated by secondary infectious foci; but this can only be accomplished by skilled investigators and in clinical laboratories. Similarly, the determina¬ tion of the amount of fibrin in the blood is a

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... er that must be left to proficient chemists. The French have taught that the quantity of fibrin may be deter¬ mined by a study of the reticulum formed under a coverglass. This teaching is certainly very far from accu¬ rate. Even if the method were reliable, the quantity of fibrin in the blood is of little practical importance. The recent studies of Pfeiffer and others have shown that there is a notably increased amount of fibrin in the blood in leucocytosis, but there are many causes of leucocytosis and there is nothing pecular to that of pneumonia, as far as its influence on fibrin is concerned. We are limited then in our study of the blood of pneu¬ monia to the ordinary clinical examinations. The points of importance that have been recognized by writers and investigators are these : In croupous pneumonia and in bronchopneumonia there is usually leucocytosis. This varies in degree and in duration. When the disease subsides, the leucocytes return to the normal number more or less quickly. There is rarely a critical1 decline in the leucocyte count comparable to that in temperature, further it is known that the leucocy¬ tosis is of the active polymorphous variety of Ehrlich, that is the actively ameboid corpuscles are increased more than the other forms-in other words, the leucocytosis is a chemotactic on'e. The eosinophilous cells are usually greatly reduced in numbers and some have claimed that they are absent. Finally, there is an undoubted relation¬ ship between the leucocyte count and prognosis, cases in which the number of white cells is normal or subnormal usually terminating fatally. These facts have all been established, and in the main my observations tend to confirm each one of them. Going back somewhat to the consideration of the nature of leucocytosis, I may state with positiveness that this condition, at least when of the forms described in pneu¬ monia, is the result of chemotactic influences. The toxic substances elaborated by the pneumococcus are positively chemotactic and attract to the circulation the ameboid polymorphous corpuscles. This, however, does not ex¬ press all of the conditions, else there would probably be leucocytosis in every case. The additional element is the reaction of the tissues themselves to the irritant influence of toxic agents. I can not now go into the matter of the pathogenesis of leucocytosis at length, but suffice it to say that experimenters have been able to demonstrate that varying systemic conditions and varying doses of toxins, such as that of the bacillus pyocyaneus, occasion different grades of leucocytosis.