276 INFECTION AND RESISTANCE 



former an emergency procedure carried out by virtue of the retained 

 ancestral characteristics of the special phagocytic cells. 



The leukocytes, whose chief functions seem to be associated with 

 such processes of intracellular digestion, may, therefore, be looked 

 upon as cells retaining primitive characteristics for definite physio- 

 logical purposes. We shall see, however, that, to meet exceptional 

 conditions, the process of phagocytosis may be carried out also by 

 many other cells which are associated ordinarily with functions en- 

 tirely apart from this phenomenon. 



During normal life in higher animals, too, constant destruction 

 of red blood cells by phagocytosis takes place in the spleen and liver, 

 and is described by Dickson 11 as occurring in the bone marrow as 

 well ; and similar phagocytosis of red cells is seen in the hemolymph 

 nodes. It is claimed by Metchnikoff, furthermore, that many of the 

 degenerative and retrogressive processes which take place in the 

 human body are carried on by the mechanism of phagocytosis. The 

 rapid return of the puerperal uterus to the normal state is explained 

 in this way, and work by Helme 12 seems to show that there is an 

 actual phagocytosis of the hyperplastic uterine musculature during 

 this period. The atrophic changes of senility, too, are attributed by 

 Metchnikoff 13 14 to the same processes. The involution of the 

 ovaries is accompanied by active phagocytosis of portions of this 

 organ, and Metchnikoff claims further to have shown that the de- 

 generation of the nervous system during old age is. accomplished by 

 the phagocytosis of nerve cells by phagocytic elements derived either 

 from the leukocytes or the neuroglia, or from both. 15 The whitening 

 of the hair, both in human beings and in old animals (dogs), is simi- 

 larly due, he claims, to phagocytosis of the pigment by cells which 

 wander in from the root sheaths. It is, up to the present time, im- 

 possible to determine the stimulus to which this phagocytosis is due. 



Since the subject is a very important one, many studies have been 

 made to determine which cells of the body of higher animals can 

 take in and digest foreign particles and to classify them according 

 to this power. Metchnikoff has distinguished between the "motile" 

 and "fixed" phagocytes, the former the leukocytes of the circulating 

 blood, the latter certain connective tissue cells, endothelial cells, 

 splenic pulp cells, and certain cellular elements of the lymph nodes, 



11 Dickson. "The Bone Marrow," Longmans, Green, London, 1908. 



12 Helme. Transact. Eoy. Soc. of Edinburgh, Vol. 35, 1889. Cited from 

 Metchnikoff. 



13 Matschinsky. Ann. de I'Inst. Past., Vol. 14, 1900. 



14 Metchnikoff. Ann. de I'Inst. Past., Vol. 15, 1901. 



15 That the leukocytes are concerned in the destruction and resorption 

 of dead tissues has been shown by Leber especially (Leber, "Die Entstehung 

 der Entziindung," Leipzig, Engelmann, 1891). An accumulation of leukocytes 

 about a bacterial focus or from any other stimulus is followed by tissue 

 lysis due to leukocytic enzymes. 



