PHAGOCYTES 231 



for the performance of various functions that adapt the individual 

 to his environment. The function of certain classes of cells is 

 secretion. Thus the cells of the alimentary tract secrete the 

 digestive ferments which prepare the food for assimilation, while 

 such glands as the thyroid and the super-renal capsules manufac- 

 ture ' internal ' secretions which pass into the blood, and are 

 essential to the well-being of the cell-community. Very important 

 members of the cell-community are certain colourless cells, the 

 white blood corpuscles or leucocytes, which float in the blood 

 stream or wander in the tissue spaces. In a sense not altogether 

 metaphorical, these cells are the sanitary inspectors, the police, the 

 scavengers of the community. They may be seen to approach 

 intrusive microbes, enclose them within their own substance, and 

 destroy them, apparently by digestion, or else perish themselves, 

 apparently by poisoning. On account of this function of ingesting 

 microbes they have been termed phagocytes. 



386. The fluid part of the blood as distinguished from the 

 corpuscles is termed serum. Several species of microbes, for 

 example those of diphtheria, flourish in serum which has been 

 withdrawn from the body and freed from blood-cells. It is, there- 

 fore, for them a nutritive, not a poisonous substance. But if 

 microbes are introduced into the body enclosed in a capillary 

 glass tube, the ends of which are plugged by a substance that 

 permits the free diffusion of fluids, but prevents the escape of the 

 micro-organisms or the entrance of the phagocytes, the latter 

 collect about the tube in numbers, especially at the open ends 

 where they form clusters. Presumably, as sanitary officials, they 

 are attracted by the secretions (toxins) of the microbes. Presently 

 the micro-organisms perish and disintegrate, apparently in much 

 the same way as when actually enclosed in the substance of the 

 phagocytes. Similarly during recovery from diphtheria the bacilli 

 may be found perishing on the surface of the throat, even when not 

 in contact with phagocytes. It appears, then, that the phagocytes, 

 and possibly other cells of the body, secrete substances, as harmless 

 to themselves as pepsin to the stomach cells, which act as counter- 

 toxins and poison the microbes, and that they are stimulated to 

 this act by the presence of the microbes and their toxins. Pre- 

 sumably these counter-toxins are identical with or similar to the 

 digestive ferments that destroy bacilli which are actually ingested 

 by the phagocytes. 



387. The toxins of the various species of microbes are not 

 identical, as is proved by the fact that the symptoms of the 



