58 THE DEFENSIVE FORCES OF THE MACROORGANISM 



to cells which are derivatives of the original mesoderm, the nerve 

 cell being the only apparent exception, on the basis at least that 

 leprosy bacilli in variable number have been encountered in these 

 cells, and assuming that their entrance occurred through the activity 

 of the nerve cell itself. All other cells in which phagocytosis has 

 been observed are mesodermal derivatives. 



Microphages and Macrophages. Metschnikoff, to whom we are 

 indebted for so much of our knowledge of phagocytosis, in all its 

 aspects, divides the cells which are endowed with this power into two 

 large groups, viz., the microphages and macrophages. The former 

 group is represented practically exclusively by the neutrophilic 

 polymorpho nuclear and polynuclear leukocytes, while the mast cells 

 and eosinophiles either do not engage in phagocytosis at all, or do so 

 only to a slight and unimportant extent. The macrophages, on the 

 other hand, comprise the large mononuclear leukocytes of the blood, 

 the endothelial cells lining the peritoneal cavity, the sessile (fixed) 

 mononuclear cells of the splenic follicles and the sinuses of the lymph 

 glands, the stellate cells of Kupffer in the liver, the large mononu- 

 clear, so-called alveolar, epithelial cells of the lungs, which latter two 

 according to Metschnikoff are in reality large mononuclear leuko- 

 cytes; and finally the bone corpuscles and the myeloplaxes or giant 

 cells of the bone-marrow. 



Phagocytic Function of Various Types of Cells. What the significance 

 of the phagocytic function of these various types of cells really is in 

 an organism in which so extensive a differentiation has taken place 

 as in the vertebrate animal is largely a subject of conjecture. We 

 may imagine, however, that under normal conditions phagocytosis 

 plays no essential role, and merely represents a general property of 

 mesoblastic protoplasm which is of interest ontogenetically, but 

 of no practical importance. But we can readily see that this same 

 function, even though it remains dormant, while the body is in 

 perfect health, immediately assumes importance of the first order, 

 if foreign cells are introduced from without. Under such conditions 

 the phagocyte is placed in a similar position as its ancestral proto- 

 type, the ameba, and it would accordingly display the same or 

 similar functions, of which the phagocytic action is one. In the 

 case of the microphages (polynuclear neutrophilic leukocytes) at 

 any rate we have no evidence that their phagocytic power enters 

 into action under normal conditions, while with the macrophages 



