HEALTH AND DISEASE 43 



by a " chill," by unsuitable food, by nicotine or alcohol, or by 

 some other poison ; or the power of resistance may be lessened 

 by other diseases, by old age or the like. 



During or after its entrance into the tissues, the bacterial or 

 other intruder has to be attacked, and weakened or killed, 

 removed from the tissues, and expelled ; and there are two 

 chief factors in this defensive process. Phagocytosis (Greek : 

 phagein, to eat, kytos, a vessel or cell) is a function of many of 

 the constituent cells of tlie animal body. Not only the 

 wandering white cells, or leucocytes of the blood, but also the 

 cells lining blood- and lymph-vessels and the serous sacs 

 around the internal organs ; the cells of the many glands of 

 the body, e.g. the liver ; the cells in connective tissues, etc. ; 

 have the power of ingesting, englobing, or engulfing foreign 

 particles such as bacteria, and digesting them. The most 

 picturesque and most easily studied of these phagocytic cells 

 are the leucocytes of the blood. The most numerous of these 

 are the polymorphonuclear leucocytes, one of whose most im- 

 portant functions is the attacking and ingesting, and if possible 

 digesting and destroying, of any invading bacteria. Certain 

 other leucocytes, the large mononuclear cells, are also very 

 important phagocytes. They too take up bacteria, but one of 

 their characteristic functions is the attacking and englobing of 

 protozoal parasites such as the malarial parasite ; and they also 

 take up and digest any of their polymorphonuclear brethren 

 that are injured in the fray. For full details of the process, 

 reference must be made to books on Pathology. Phagocytosis 

 is really a special instance of the way in which a simple cell 

 such as a protozoon (e.g. the amoeba) obtains its food. It 

 approaches its prey, drawn by some subtle and as yet little- 

 understood influence, to which the name chemiotaxis has been 

 given, and may paralyse or otherwise a fleet it by means of 

 ferments which it secretes. It then pushes out around it arm- 

 like processes of its protoplasm called pseudopodia (Greek: 

 pseudes, false, pous, a foot), draws its prey within its cell-sub- 

 stance, and proceeds to digest it by means of ferments. 



Besides these phagocytic cells, there are, in the blood and 

 other body fluids, innumerable chemical substances, many of 

 them of the nature of ferments, which are elaborated by the 

 various cells for the purpose, amongst other things, of affecting 

 any bacteria or other hostile cells which may have gained 

 entrance. Many of these substances are normally present in 

 the healthy animal, but a large number of them are specially 

 developed only after the hostile agent has gained an entrance, 

 and are specific only against the special variety of bacterium or 

 protozoon concerned. The new-born child inherits a certain 



