MACROPHAGES FROM ARACHNOID CELLS. 385 



membrane. If blood is absolutely confined to the subdural space, the layer of 

 arachnoidal cells facing the dura exhibits the characteristics welling-up with macro- 

 phage formation, while the layer of cells lining the subarachnoid space, although 

 separated from those of the subdural space by only a very thin layer of white 

 fibrous tissue, remains entirely unaffected. Vice versa, the arachnoid cells compos- 

 ing the membrane looking toward the dura never participate in the reaction towards 

 a stimulus applied to the subarachnoid space. The collection of cells forming the 

 plaques noted above never has been seen to furnish macrophages or even to phago- 

 cytize particles of matter. 



Permanent specimens stained with the ordinary methods furnish little addi- 

 tional information. Quite often we have all of the inclusions of the cells dissolved 

 out or not counterstained. This gives us a chance to study the protoplasm of the 

 cells as they rest on the trabeculse. The typical foamy structure occurring in the 

 macrophages is shown in figures 2b and 2c. As ordinarily seen (figs. 9 and 10) this 

 network is the result of the vacuoles and ingested material within the cell. The 

 loose meshwork has been frequently misinterpreted as a sign of degeneration, but 

 it certainly suggests, in these experiments, hungry active cells. 



A very natural question arises concerning the fate of the denuded trabeculse 

 and those regions of the membrane which have lost likewise their covering of 

 cells. It is quite easy to convince one's self that the number of cells covering the 

 connective-tissue strands is reduced or, in specific instances, that the entire cellular 

 covering of the trabeculse is gone. Carelessness in preparing the specimen may 

 result in a flaking off of the cells clothing the trabeculse, and undue tension brings 

 about a loosening of the cells or even produces a naked bundle of fibrous tissue. 

 On the other hand, the number of cells per unit of area is by no means constant in 

 the normal arachnoid, and the personal element of interpretation can be exercised 

 to any extent. The efforts to replace cells which have budded off are widely dis- 

 tributed at the time when the production of the free-moving macrophages is at its 

 height. Mitotic figures occur with marked frequency, both among the cells covering 

 the trabeculse (fig. 5) and the membranous expansion of the arachnoid (fig. 11). 

 The localized occurrence of dividing cells corresponds to the areas of particulate 

 stimulation, and in all probability there never exists a true denuding of the surface. 

 During the process of division the protoplasm takes on a denser stain, partaking 

 more strongly of the basic dyes. The protoplasmic bodies of the cells which have 

 not detached themselves close over the gap and very shortly a new division takes 

 place. An actual proliferation of arachnoid cells, resulting in the formation of a 

 regular morula mass, has not been observed in connection with blood stimulation 

 alone, but the combination of infection and blood gave a remarkable picture of this 

 phenomenon. 



Experiments were carried out with dilute suspensions of carbon and cinnabar 

 granules. Phagocytosis of inert matter by the cells comprising the membrane 

 could not be expected to be as vigorous as is the case with erythrocytes, inasmuch 

 as the stimulant may be slightly toxic and in no wise can be used for food. Removal 

 of the last traces of such particles involves months. The actual production of 



