[24] From a paper of Calleja we learn that Ramon y Cajal recognised the halos of the mast cells, and interpreted them in the manner we have above. Calleja also describes these halos and the method of demonstrating them in detail (thionin staining, and mounting the sections in glycerine). We must mention, however, that we do not consider this method suitable for the recognition of preformed halos, for the reasons above mentioned.
The problem of leucocytosis is one of the most keenly debated questions of modern medicine. An exhaustive account of the various works devoted to it, of the methods and results, could fill by itself a whole volume, and would widely exceed the limits of an account of the histology of the blood. We can only deal fully therefore with the purely hæmatological side of the subject.
Virchow designated by the name "=Leucocytosis=," a transient increase in the number of the leucocytes in the blood; and taught that it occurred in many physiological and pathological conditions. In the period that followed particular attention was paid to the leucocytosis in infectious diseases, and to the investigators of the last 15 years in this province we owe very important conclusions as to the ~biological meaning~ of this symptom. Above all Metschnikoff has done pioneer service in this direction by his theory of phagocytes, and though his theory has been shaken in many essential points, yet it has exercised a stimulating and fruitful influence on the whole field of investigation.
To sketch Metschnikoff's doctrine in a few strokes is only possible by a paraphrase of the very pregnant words "Phagocytes, digestive cells." These words express the view, that the leucocytes defend the organism against bacteria by imprisoning them by the aid of their pseudopodia, taking them up into their substance, and so depriving them of the power of external influence. The issue of an infectious disease would chiefly depend on whether the number of leucocytes in the blood is sufficient for this purpose.
This engaging theory of Metschnikoff has undergone important limitations as the result of further investigation. Denys, Buchner, Martin Hahn, Goldscheider and Jacob, Löwy and Richter, and many others have demonstrated, that the most important weapon of the leucocytes is not the mechanical one of their pseudopodia, but their chemical products ("Alexine," Bucher). By the aid of bactericidal or antitoxic substances which they secrete, they neutralise the toxines produced by the bacteria, and thus render the foe harmless by destroying his weapon of offence, even if they do not exterminate him.