SUSCEPTIBILITY AND IMMUNITY. 259 



splenic pulp. But there are also cells as, for instance, those forming the 

 endothelial lining- of the vessels which are very definitely fixed, which 

 nevertheless can give off protoplasmic processes from their free surface and 

 so capture and include bacteria. 



All these may be spoken of as phagocytes, and may be divided into the 

 two broad groups affixed phagocytes (endothelial cells, etc.) and free (leu- 

 cocytes). Not that the terms "phagocyte" and "leucocyte" are synonymous, 

 for of the latter three main forms may be distinguished, of which one is 

 practically immobile and never takes up bacteria. This is the lymphocyte, 

 characterized by its relatively small size, its large single nucleus, and the 

 small amount of surrounding protoplasm. The two remaining (phagocytic) 

 forms are, first, the large uninuclear leucocyte, whose prominent nucleus is 

 at times lobed or reniform, which stains well with aniline dyes and possesses 

 much protoplasm and active amoeboid movements the macrophage and, 

 second, the microphage, a small form, also staining well, but either multi- 

 nuclear or with one nucleus in the process of breaking up. If now we com- 

 pare the endothelial cells with these, it is evident that their properties con- 

 nect them closely with the macrophage ; and, in fact, there is now little or 

 no doubt that a very large proportion of the macrophages are of endothelial 

 origin. 



Leaving aside the subject of amoeboid microbes and their life within ani- 

 mal cells, it is to the phagocytes and their relation to the bacteria that I wish 

 specially to draw your attention. 



Taking as wide a view as possible of this relationship, we can first deter- 

 mine that the more malignant the microorganism the rarer is its presence 

 within the phagocyte. Thus in those which of all diseases are the most 

 rapidly fatal in chicken cholera affecting birds and rabbits, in hog cholera 

 ("cholera des pores") given to pigeons and rabbits, in the anthrax of mice 

 and other specially sensitive animals, in the "septicemie vibrionienne" of 

 guinea-pigs and birds, and in yet other diseases of peculiarly swift course 

 the corresponding bacteria are only very exceptionally to be found within 

 the cells, but remain free in the neighborhood of their introduction and 

 thence invade the blood. For all the above-mentioned diseases are not 

 localized, but, on the contrary, present the characters of general acute sep- 

 ticaemia, causing death within twenty to thirty-six hours, or, in certain 

 cases, even within six hours. 



And when we pass to those diseases in which the bacteria are to be found 

 either in part or almost wholly within the phagocytes, the same law still 

 applies ; for in such cases the disease has lost its suddenness, tending to 

 have a slower course, or, indeed, to be of a chronic nature. Even in those 

 affections in which an acute course is accompanied by considerable phago- 

 cytosis, the fatal termination is far from occurring at the same early period 

 as in the diseases recorded above. Thus mouse septicaemia, characterized as 

 it is by frequent intracellular bacteria, has a duration in the mouse two and 

 a half times as long as that of anthrax in the same animal. But in general a 

 well-marked phagocytosis is associated with diseases presenting an essen- 

 tially chronic development ; it is in affections such as tuberculosis, leprosy, 

 rhinoscleroma, glanders, that the specific bacteria are most readily taken up 

 by the phagocytes ; it is here that, at the seat of the disease, we meet with in- 

 numerable macrophages epithelioid cells in which lie the individual micro- 

 organisms. 



Further, if we consider the phenomena associated with the resolution of 

 an infectious disease, this inverse relationship between the malignancy of the 

 malady and the occurrence of phagocytosis is, if possible, yet more clearly 

 demonstrated. Notice, for instance, what obtains during the progress of re- 

 lapsing fever, a malady still fairly common in Russia and other Sclavonic 

 countries, and one which, while presenting many difficulties to the bacteri- 

 ologist, in that the specific spirochaete has so far resisted cultivation, and in 

 that it cannot be communicated to the ordinary animals of the laboratory, is 

 nevertheless in many respects not ill -adapted for our present purpose. Here, 



